Evaluation of off-ramp right turn control at single point urban interchanges without frontage roads

              EVALUATION OF OFF-RAMP RIGHT
TURN CONTROL AT SINGLE POINT
URBAN INTERCHANGES WITHOUT
FRONTAGE ROADS
Final Report 556
Prepared by:
Jim C. Lee
Brennan D. Kidd
Lee Engineering, LLC
3033 N. 44th Street, Suite 375
Phoenix, AZ 85018
James A. Bonneson
Karl Zimmerman
Texas Transportation Institute
College Station, Texas 77843-3135
January 2006
Prepared for:
Arizona Department of Transportation
206 S. 17th Avenue
Phoenix, Arizona 85007
In cooperation with
US Department of Transportation
Federal Highway Administration
DISCLAIMER
The contents of this report reflect the views of the authors who are responsible for the
facts and the accuracy of the data presented herein. The contents do not necessarily
reflect the official views or policies of the Arizona Department of Transportation or the
Federal Highway Administration. This report does not constitute a standard,
specification, or regulation. Trade or manufacturers' names which may appear herein are
cited only because they are considered essential to the objectives of the report. The U.S.
Government and the State of Arizona do not endorse products or manufacturers.
Technical Report Documentation Page
1. Report No.
FHWA-AZ-06-556
2. Government Accession
No.
3. Recipient's Catalog No.
5. Report Date
JANUARY, 2006
4. Title and Subtitle
EVALUATION OF OFF-RAMP RIGHT TURN CONTROL AT
SINGLE POINT URBAN INTERCHANGES WITHOUT
FRONTAGE ROADS
6. Performing Organization Code
7. Author
Jim C. Lee, Brennan D. Kidd, James A. Bonneson,
Karl Zimmerman
8. Performing Organization Report No.
9. Performing Organization Name and Address 10. Work Unit No.
Lee Engineering, LLC
3033 N. 44th Street, Suite 375
Phoenix, AZ 85018
11. Contract or Grant No.
SPR-PL-1(63) 556
13.Type of Report & Period Covered
Final Report, 8/03-1/06
12. Sponsoring Agency Name and Address
Arizona Department of Transportation
206 S. 17th Avenue
Phoenix, Arizona 85007
Project Manager: Yongqi Li
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared in cooperation with the U.S. Department of Transportation, Federal Highway Administration
16. Abstract
This study focused on the control of the off-ramp right turn movement at SPUIs without frontage roads. The
objective of this research project was to evaluate the safety and efficiency of traffic control for off-ramp right turns.
For the purposes of this project, two common forms of off-ramp right turn traffic control were investigated: signal
control and yield control.
The process followed during this research focused on two main aspects of the off-ramp right turn movement: safety
and operations. The project was composed of the following stages: literature review, safety analysis and operation
analysis. Literature Review: A literature review was conducted to provide the research team a broader perspective on
other studies concerned with this aspect of SPUIs. The review was looking for the various traffic controls and
interchange configurations that could particularly affect the safety and operation efficiency of off-ramp right turn
movement. Safety Analysis: Long-term trends in crash occurrences and short-term observations of conflicts at six
study sites (12 off-ramp locations) were analyzed. Crash rates and conflict rates were determined in order to compare
and contrast the two means of assessing safety as well as how they relate to the type of the traffic control used at the
off-ramps. Operations Analysis: Detailed traffic data collected at the study sites was used to calculate actual delays for
off-ramp right turn movements at the study sites. This field data was also used to conduct simulations of interchange
which supplemented the calculations based on the limited sample of study sites. The simulation models provided a
means of testing different combinations of off-ramp right turn control types and overall interchange conditions in order
to determine the effects of signal and yield control.
17. Key Words
Interchange, single-point, SPUI, off-ramp right turn, signal
control, yield control, crashes, interchange safety, costs,
interchange operation
18. Distribution Statement
Document is available to the
U.S. public through the National
Technical Information Service,
Springfield, Virginia, 22161
19. Security Classification
Unclassified
20. Security Classification
Unclassified
21. No. of Pages
161
22. Price
23.
Registrant's
Seal
SI* (MODERN METRIC) CONVERSION FACTORS
APPROXIMATE CONVERSIONS TO SI UNITS APPROXIMATE CONVERSIONS FROM SI UNITS
Symbol When You Know Multiply By To Find Symbol Symbol When You Know Multiply By To Find Symbol
LENGTH LENGTH
in Inches 25.4 millimeters mm mm millimeters 0.039 inches in
ft Feet 0.305 meters m m meters 3.28 feet ft
yd Yards 0.914 meters m m meters 1.09 yards yd
mi Miles 1.61 kilometers km km kilometers 0.621 miles mi
AREA AREA
in2 square inches 645.2 square millimeters mm2 mm2 Square millimeters 0.0016 square inches in2
ft2 square feet 0.093 square meters m2 m2 Square meters 10.764 square feet ft2
yd2 square yards 0.836 square meters m2 m2 Square meters 1.195 square yards yd2
ac Acres 0.405 hectares ha ha hectares 2.47 acres ac
mi2 square miles 2.59 square kilometers km2 km2 Square kilometers 0.386 square miles mi2
VOLUME VOLUME
fl oz fluid ounces 29.57 milliliters mL mL milliliters 0.034 fluid ounces fl oz
gal Gallons 3.785 liters L L liters 0.264 gallons gal
ft3 cubic feet 0.028 cubic meters m3 m3 Cubic meters 35.315 cubic feet ft3
yd3 cubic yards 0.765 cubic meters m3 m3 Cubic meters 1.308 cubic yards yd3
NOTE: Volumes greater than 1000L shall be shown in m3.
MASS MASS
oz Ounces 28.35 grams g g grams 0.035 ounces oz
lb Pounds 0.454 kilograms kg kg kilograms 2.205 pounds lb
T short tons (2000lb) 0.907 megagrams
(or “metric ton”)
mg
(or “t”)
Mg megagrams
(or “metric ton”)
1.102 short tons (2000lb) T
TEMPERATURE (exact) TEMPERATURE (exact)
ºF Fahrenheit
temperature
5(F-32)/9
or (F-32)/1.8
Celsius temperature ºC ºC Celsius temperature 1.8C + 32 Fahrenheit
temperature
ºF
ILLUMINATION ILLUMINATION
fc foot candles 10.76 lux lx lx lux 0.0929 foot-candles fc
fl foot-Lamberts 3.426 candela/m2 cd/m2 cd/m2 candela/m2 0.2919 foot-Lamberts fl
FORCE AND PRESSURE OR STRESS FORCE AND PRESSURE OR STRESS
lbf Poundforce 4.45 newtons N N newtons 0.225 poundforce lbf
lbf/in2 poundforce per
square inch
6.89 kilopascals kPa kPa kilopascals 0.145 poundforce per
square inch
lbf/in2
SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380
TABLE OF CONTENTS
EXECUTIVE SUMMARY .............................................................................................. 1
CHAPTER 1 LITERATURE REVIEW: OFF-RAMP RIGHT TURN CONTROL
AT SINGLE POINT URBAN INTERCHANGES (SPUI) WITHOUT FRONTAGE
ROADS.............................................................................................................................. 5
OVERVIEW................................................................................................................... 5
OPERATIONAL ASPECTS .......................................................................................... 6
General Characteristics of Off-Ramp Right Turns ..................................................... 6
Free/Uncontrolled Off-Ramp Right Turns.................................................................. 7
Controlled Off-Ramp Right Turns.............................................................................. 9
SAFETY ASPECTS ..................................................................................................... 12
PEDESTRIAN & BICYCLIST SAFETY.................................................................... 12
Vehicular Safety........................................................................................................ 13
CONCLUSIONS........................................................................................................... 14
CHAPTER 2 EXISTING CONDITIONS AT STUDY SITES: OFF-RAMP RIGHT
TURN CONTROL AT SINGLE POINT URBAN INTERCHANGES (SPUI)
WITHOUT FRONTAGE ROADS................................................................................ 17
INTRODUCTION ........................................................................................................ 17
DATA COLLECTION EFFORT ................................................................................. 17
Study Site Selection .................................................................................................. 17
Operations-Related Data........................................................................................... 27
Safety-Related Data .................................................................................................. 34
CALCULATIONS........................................................................................................ 42
Control Delay Calculations....................................................................................... 43
Conflict Rate Calculations ........................................................................................ 45
Crash Rate Calculations............................................................................................ 47
CONCLUSIONS........................................................................................................... 47
General Operations ................................................................................................... 47
Control Delay............................................................................................................ 49
Conflict Rate Comparison......................................................................................... 50
Crash Rate Comparison ............................................................................................ 50
CHAPTER 3 OPERATIONAL ASSESSMENT OF DIFFERENT TYPES OF OFF-RAMP
RIGHT TURN CONTROL AT SINGLE POINT URBAN
INTERCHANGES (SPUI) WITHOUT FRONTAGE ROADS.................................. 53
INTRODUCTION ........................................................................................................ 53
CALIBRATION OF CORSIM MODEL...................................................................... 53
Modeling of Off-Ramp Right Turn Lanes ................................................................ 54
CORSIM Parameters and Distributions.................................................................... 55
Field Data Inputs....................................................................................................... 56
CALIBRATION RESULTS......................................................................................... 58
Delay......................................................................................................................... 58
Green Phase Duration ............................................................................................... 59
Percentage of Off-Ramp Right Turn Vehicles Stopping .......................................... 60
Analysis of Off-Ramp Right Turn Control Types .................................................... 61
Off-Ramp Right Turn Control Types ....................................................................... 61
Creation of Model Base Files ................................................................................... 62
Determination of Volume Conditions to Analyze .................................................... 63
Model Simulation Process ........................................................................................ 64
Simulation Results .................................................................................................... 64
CHAPTER 4 CONCLUSIONS...................................................................................... 67
INTRODUCTION ........................................................................................................ 67
SAFETY ....................................................................................................................... 67
Crash History Analysis ............................................................................................. 67
Conflict Observations & Analysis ............................................................................ 68
Safety Conclusions.................................................................................................... 69
OPERATIONS.............................................................................................................. 69
Field Measurements & Calculations......................................................................... 69
Model/Simulation Analysis ...................................................................................... 71
Operations Conclusions ............................................................................................ 71
OVERALL CONCLUSIONS....................................................................................... 72
Crash Costs ............................................................................................................... 72
Operations Costs ....................................................................................................... 73
IMPLEMENTATION................................................................................................... 75
REFERENCES................................................................................................................ 77
APPENDIX A – Raw Traffic Volume Data Collected................................................. 79
APPENDIX B – Control Delay Calculation Description & Example ...................... 149
LIST OF TABLES
Table 1: SPUI Study Sites Characteristic Data ........................................................26
Table 2: SR 51/Indian School Road –
Southbound Off-Ramp Right Turn Related Crashes .................................37
Table 3: SR 51/Indian School Road –
Northbound Off-Ramp Right Turn Related Crashes .................................37
Table 4: SR 51/Glendale Avenue –
Southbound Off-Ramp Right Turn Related Crashes .................................37
Table 5: SR 51/Glendale Avenue –
Northbound Off-Ramp Right Turn Related Crashes .................................38
Table 6: SR 51/Cactus Road –
Southbound Off-Ramp Right Turn Related Crashes .................................38
Table 7: SR 51/Cactus Road –
Northbound Off-Ramp Right Turn Related Crashes .................................38
Table 8: SR 51/Greenway Road –
Southbound Off-Ramp Right Turn Related Crashes .................................39
Table 9: SR 51/Greenway Road –
Northbound Off-Ramp Right Turn Related Crashes .................................39
Table 10: SR 51/Bell Road –
Southbound Off-Ramp Right Turn Related Crashes .................................39
Table 11: SR 51/Bell Road –
Northbound Off-Ramp Right Turn Related Crashes .................................40
Table 12: Loop 202/Rural Road –
Westbound Off-Ramp Right Turn Related Crashes ..................................41
Table 13: Loop 202/Rural Road –
Eastbound Off-Ramp Right Turn Related Crashes....................................42
Table 14: Control Delay for Off-Ramp Right Turn Movements ...............................44
Table 15: Conflict Data and Rate Computations for
Off-Ramp Right Turn Movements.............................................................45
Table 16: Crash Data and Rate Computations for
Off-Ramp Right Turn Movements ............................................................46
Table 17: Comparison of Simulation Results for
Off-Ramp Right Turn Control Type Scenarios .........................................65
Table 18: Calculated Peak Hour Control Delay for Off-Ramp Right Turn
Movements at Study Sites..........................................................................70
Table 19: Summarized Crash Data and Estimated Annual Costs
at the Study Sites........................................................................................72
Table 20: Summarized Operations Data and Estimated Annual Costs
at the Study Sites........................................................................................74
LIST OF FIGURES
Figure 1: Common Right Turn Lane Configurations at Exit Ramps ..........................8
Figure 2: Signal Head Placement for Exit Ramp Right Turns ..................................10
Figure 3: Study Sites .................................................................................................18
Figure 4: State Route 51 & Indian School Road Aerial Photograph.........................20
Figure 5: State Route 51 & Glendale Avenue Aerial Photograph.............................21
Figure 6: State Route 51 & Cactus Road Aerial Photograph ....................................22
Figure 7: State Route 51 & Greenway Road Aerial Photograph...............................23
Figure 8: Loop 101 (Agua Fria Freeway) & Bell Road Aerial Photograph..............24
Figure 9: Loop 202 (Red Mountain Freeway) & Rural Road Aerial Photograph.....25
Figure 10: Existing 2004 Volumes – SR 51 & Indian School Road...........................28
Figure 11: Existing 2004 Volumes – SR 51 & Glendale Avenue...............................29
Figure 12: Existing 2004 Volumes – SR 51 & Cactus Road ......................................30
Figure 13: Existing 2004 Volumes – SR 51 & Greenway Road.................................31
Figure 14: Existing 2004 Volumes – Loop 101 & Bell Road .....................................32
Figure 15: Existing 2004 Volumes – Loop 202 & Rural Road...................................33
Figure 16: Schematic Conflict/Crash Location Key Map ...........................................36
Figure 17: Reassignment Phases for Loop 202/Rural Road Interchange....................57
Figure 18: Comparison of Simulated and Field-Measured Delays
for the Off-Ramp Right Turn Movement...................................................58
Figure 19: Comparison of Simulated and Field-Measured Green Phase Durations
for the Off-Ramp Right Turn Movement...................................................59
Figure 20: Comparison of Simulated and Field-Measured Percentage of
Vehicles Stopping for the Off-Ramp Right Turn Movement ....................60
1
EXECUTIVE SUMMARY
INTRODUCTION
Single point urban interchanges (SPUIs) have become an integral part of managing traffic
at the critical connections between freeway and arterial roadway systems. Although
studies and debates continue as to where and how they should be applied, they do not
discount their continued application. Based on this more widespread use, finer aspects of
their operation are being considered and studied. This study focused on the control of the
off-ramp right turn movement at SPUIs without frontage roads. The objective of this
research project was to evaluate the safety and efficiency of traffic control for off-ramp
right turns. For the purposes of this project, two common forms of off-ramp right turn
traffic control were investigated: signal control and yield control.
SCOPE OF RESEARCH
The process followed during this research focused on two main aspects of the off-ramp
right turn movement: safety and operations. The project was composed of the following
stages:
Literature Review: A literature review was conducted to provide the research team a
broader perspective on other studies concerned with this aspect of
SPUIs. The review was looking for the various traffic controls and
interchange configurations that could particularly affect the safety
and operation efficiency of off-ramp right turn movement.
Safety Analysis: Long-term trends in crash occurrences and short-term observations
of conflicts at six study sites (12 off-ramp locations) were
analyzed. Crash rates and conflict rates were determined in order
to compare and contrast the two means of assessing safety as well
as how they relate to the type of the traffic control used at the off-ramps.
Operations Analysis: Detailed traffic data collected at the study sites was used to
calculate actual delays for off-ramp right turn movements at the
study sites. This field data was also used to conduct simulations of
interchange which supplemented the calculations based on the
limited sample of study sites. The simulation models provided a
means of testing different combinations of off-ramp right turn
control types and overall interchange conditions in order to
determine the effects of signal and yield control.
2
FINDINGS
The review of relevant literature and research shows that there is some attention devoted
to the operation and safety of SPUIs specifically pertaining to the off-ramp right turn
movement. The literature review also revealed that there does not appear to be any past
or present research/studies investigating the advantages and disadvantages of using one
form of control over another for the off-ramp right turn movement. Most of the
information reviewed pertained to the advantages and disadvantages of free/uncontrolled
off-ramp right turn movements versus some type of control (i.e., stop sign, yield, or
signal). Key concepts relating to the types of off-ramp right turn control that were
discovered during the literature review and considered throughout the research included
the effect of nearby downstream intersections, pedestrian/bicyclist activity at the
interchange, increased clearance intervals with signal control, and other issues further
discussed within the report.
The data collection effort and details obtained from observations and research allowed for
actual calculations to be made concerning operations and safety. Interpretation of that
data through the results of the calculations lends itself to determining interchange
characteristics that influence operations and/or safety, but is subject to the limited number
(6) of study interchanges evaluated. Qualitative observations and conclusions regarding
the operations and safety of the study interchanges are presented within this report.
Delays, conflict rates, and crash rates were calculated from the data and observations at
the six study sites. Average delays for off-ramp right turn vehicles at signal- controlled
locations experienced about 20% to 30% more delay than the vehicles at locations with
yield control. The overall conflict rates for the control-type groups were based on a
recalculation of the conflict rate using the summed values for each sample site. An
overall average of the crash rates calculated for each site was not deemed appropriate
given the variability inherent to conflict observations based on the relatively short
observation period as compared to crash rate calculations. The average conflict rate for
the yield-controlled sites as a group is about 240% greater than the average rate for the
signal-controlled group, but the yield-controlled sites have considerable variability in
their rates. A statistical t-test indicates that because of this variability and despite the
large difference in average rates, there is no significant difference (tcalc = 1.705, t.05, v=10 =
1.812) in the average conflict rates between the control groups. Overall crash rates for the
control-type groups were the averaged values of the three-year average crash rate for
each site in the group. The average crash rate for yield-controlled sites as a group is
almost double the average crash rate for the signal-controlled sites. This ratio is
comparable to the conflict rate relationship between the two groups. A statistical t-test
was performed on the average crash rate data for the yield-controlled sites and the signal-controlled
sites. All crash rates were considered, which resulted in no significant
difference (tcalc = 1.510, t.05, v=10 = 1.812) in the average rates for each group.
The actual field data from the limited sample of study interchanges was supplemented
with model simulation results that considered four control type scenarios—two variations
on signal control and two on yield control. The signal control variations concern the
3
allotment of signal phasing to the off-ramp right turn traffic. One version only gives a
green arrow indication to the off-ramp right turn movement during the adjacent cross
street left turn phase. This was referred to as “Signal 1-phase” within this report. The
other variation of the off-ramp right turn signal control type is when there are two phases
that can provide the green arrow indication for the off-ramp right turn movement. This
control variation is referred to as “Signal 2-phase” in this report.
The yield control type was split into two versions incorporating vehicle presence
detection or just the standard yield sign with no vehicle detection. The off-ramp right
turn control that uses yield signs and vehicle detection works similarly to the Signal 1-
phase control, but without the signal head indications for the off-ramp right turn vehicles.
Essentially the off-ramp right turn traffic would be acting as pseudo cross street left turn
traffic. In this report, this control type is called “Yield With Detection.”
An iterative analysis process involving a range of off-ramp and interchange volume
conditions was used to determine overall operational effectiveness of each control
scenario. Data collected at several SPUI sites was used to calibrate a micro-simulation
model (CORSIM) that was then used to evaluate numerous combinations of traffic
volume conditions and off-ramp control types that would have not been possible to
collect at actual SPUI locations. The results of the simulations were used in concert with
the safety evaluation and conclusions to develop suggestions on appropriate control types
for the off-ramp right turn movement.
The results indicated that in almost all volume scenarios, the “Yield Without Detection”
control type (the basis for the comparisons) has the lowest overall interchange control
delay. When comparing averaged interchange control delays, the other control type
variations resulted in more delay. In the scenarios with one off-ramp right turn lane, the
overall interchange delay for the “Yield With Detection” and “Signal 1-Phase” were not
much greater (about 4 and 9 % more, respectively). The differences in interchange delay
were more prominent in the two-lane off-ramp right turn scenarios due to modeling
constraints, which caused the left hand lane of the two lane off-ramp right turn to
experience more delay than necessary in the scenarios with signal control. Therefore, the
magnitudes of the percent differences for the signal control types in this two-lane group
of scenarios are exaggerated, yet they still reflect the same general relationship as the
one-lane group of scenarios. Also, note that these percent differences apply for the
normal ranges of interchange volumes and turning movements used in this project.
Unusual situations may result in different results for each control type.
The efforts executed during this project had the goal of determining which control type
would be best to use for off-ramp right turn movements at single-point urban
interchanges without frontage roads. The data collected, both in the field and through the
crash databases, were very detailed, beneficial, and used to their fullest. However,
despite the efforts and underlying goal, the results from the safety and operations
analyses appear to be contrary making it necessary to compare the two aspects using a
common basis. Safety and operation can be measured in the common term of cost.
Estimates of the overall yearly costs of operations and crashes associated with the off-
4
ramp right turn movement at yield and signal-controlled site were computed as a final
means of determining the best control type.
The crash cost for each interchange is calculated from the number of crashes associated
with the off-ramp right turn movement only. Thus, the total crash cost values are not
representative of the total crash costs per interchange, but are valid for use in the
comparison against interchange operational costs since the unknown crash cost
component is assumed to be equal for all the interchanges. The costs are composed of
several factors: medical costs, property damage loss, lost productivity (market and
household), and other related costs. The average costs for crashes involving property
damage only was $4,812 (in 2004 dollars). Crashes involving injuries of varying degrees
have an average cost of $49,817. Crashes with any fatalities, which are about 75 times
less likely to occur as other injury crashes, have an average cost of $1,184,885 associated
with them. The average yearly cost of crashes for the study interchanges, grouped by off-ramp
right turn control type, indicates that interchanges using yield control for the off-ramp
right turn movement are about $384,000 (2004 dollars) more costly than the
interchanges using signal control.
The user cost aspect considered in this project was the “value of time” (user delay costs),
which accounts for a majority of the user costs in this project’s comparison of the control
types for off-ramp right turn movements. The value of time is a function of the average
hourly wage earned by the persons impacted by the delays (separated by passenger
vehicles and trucks), the percentage of the hourly wage considered as the value of time
(50% for passenger vehicles, 100% for trucks), and the average passenger occupancy (1.5
for passenger vehicles, 1.05 for trucks). The average yearly cost of delay for the study
interchanges, grouped by off-ramp right turn control type, indicates that interchanges
using signal control for the off-ramp right turn movement are about $689,000 more
costly.
For use in this comparison only, the total average yearly costs (crash costs + delay costs)
for interchanges using signal control for the off-ramp right turn movement is estimated at
$2,100,000. Interchanges that have yield control for the off-ramp right turn movement
have an average yearly cost estimate of $1,800,000. Despite yield control sites appearing
to have higher crash rates (although not statistically significant), their overall savings in
user cost of delay offsets the increased costs of crashes. However, the difference in total
costs does not appear to be substantial, at least not to a degree where the selection of a
certain control type would be more convincing than the other.
5
CHAPTER 1
LITERATURE REVIEW:
OFF-RAMP RIGHT TURN CONTROL AT
SINGLE POINT URBAN INTERCHANGES (SPUI) WITHOUT
FRONTAGE ROADS
OVERVIEW
Although there are extensive studies concerning the effectiveness of single point urban
interchanges (SPUIs), especially when compared to other interchange designs, most of
this research has focused on the overall operation and safety of the interchange types.
However, this investigation did not locate any past or current research specifically
focused on the traffic control of the right turn movement from the major roadway
associated with the SPUI and how it relates to operation and safety. The literature review
did discover there are limited publications guidelines and protocols for how this
movement should be controlled in specific conditions.
The SPUI has a unique characteristic, as compared to some other interchanges or
intersection designs, where the major roadway right turn movement (hereafter referred to
as the “off-ramp” right turn) can be accommodated by a dedicated right turn lane (or
lanes) that could be operated without any traffic control (e.g., stop, yield, or signal). In
this particular case, the off-ramp right turn is merged into the cross street traffic via a
separate additional lane on the cross street. NCHRP Report 345: Single Point Urban
Interchange Design and Operations Analysis by Messer, et al [1] found in its 1989 field
survey that only about 25% of the SPUIs were designed to accommodate a “free” off-ramp
right turn movement with a separate acceleration lane along the cross road. This
layout for the off-ramp right turn is usually permissible based on the interchange
operations, but is not always feasible. The report also states that right turns from the off-ramps
are operationally more complex and typically have less capacity per lane.
Without a “free” (uncontrolled) situation, the off-ramp right turn movement has to be
governed by some form of traffic control. The most common means of traffic control in
these situations are stop control, signal control, merge (with yield), or yield control,
which is the most prevalent [1]. The merge-type control is similar to the free right turn
discussed above except that a separate additional lane is not provided to receive the off-ramp
right turn traffic—instead a short acceleration or drop lane is provided necessitating
a yield condition at the merge point. Stop control, yield without a merge situation, and
signal control are typically implemented at the point where the right turn lane (or
curvature of the right turn lane) starts to intersect with the cross street travel lanes. Yield
control and signal control are the focus of this literature search and research as a whole.
6
OPERATIONAL ASPECTS
Several of the sources examined in this review provided information on off-ramp right
turn control as it related to operational characteristics and effects. Much of the
information focused on the advantages and disadvantages of a free/uncontrolled off-ramp
right turn versus a controlled situation (e.g., signal or yield/stop control). Although this
particular interest is different from the purpose of this research, it does provide some
insight into the benefits of one control type over another.
General Characteristics of Off-Ramp Right Turns
There are several components to the design and operation of the off-ramp right turn
movement that are independent of the type of traffic control employed. NCHRP 345 [1]
points out a few of these. Geometrically speaking, some overall characteristics that affect
off-ramp right turn operations are the magnitude of the turn radius, the presence of an
auxiliary acceleration lane at the end of the turn, and whether the off-ramp right turn lane
is exclusive. Larger turn radii can promote better off-ramp right turn operations, but at
the cost of making the movement more complex and requiring more space. Locations
where the off-ramp left turns and right turns do not have exclusive lanes will be
inefficient due to the difference in traffic controls (i.e., the respective turn lane queues
may block one another), as well as when both movements are signalized.
NCHRP 345 [1] mentions some factors that determine how well an off-ramp right turn
movement operates, what its capacity limit is, and its safety. The characteristics include
the geometry of the turn path, complexity of the entrance maneuver, capacity of the
maneuver, and type of traffic control in place. The report continues by stating, “[the]
right turn maneuver is significantly affected by the type of traffic control, e.g., stop, yield,
etc., the number of conflicting signalized movements, and the signal timing of the
conflicting movements.” (p. 24)
The complexity of the entrance maneuver can affect the efficiency and safety of the off-ramp
right turn operations. One point of complexity involves the off-ramp right turn
driver’s perception of potential conflicting traffic. Due to the signal phasing used at
SPUIs, off-ramp right turn traffic is faced with alternating sequences of high and low
traffic flows where they enter the cross street. This is not all that uncommon at
interchanges/intersections, but the distances related to a SPUI layout complicate the
decision for the driver. Another characteristic mentioned in NCHRP 345 [1] that
complicates the off-ramp right turn movement is the angle of entry and physical
requirements necessary to confirm a safe point to enter the cross street traffic stream.
The capacity of an off-ramp right turn movement is dependent on the type of traffic
control used. According to NCHRP 345 [1], if a stop or yield control is in place, the off-ramp
right turn capacity is dependent on the availability of gaps in the conflicting traffic
stream (with most of them being generated artificially by the overall SPUI signal
operations). Capacity at signal controlled off-ramp right turn movements is based on the
portion of the overall SPUI signal cycle length devoted to the off-ramp right turn
movement plus available gaps for right turn on red.
7
Free/Uncontrolled Off-Ramp Right Turns
The California Single Point Interchange Planning, Design, and Operations Guidelines
[2] mentions off-ramp right turn movements with free control. The Guidelines claims
that “free right turn moves at the exit ramps are a basic feature of the typical SPI [i.e.,
SPUI]. Lack of a free right can negatively impact operational efficiency.” (p. 9)
California views the use of SPUIs (SPIs) as a means to move large volumes of traffic,
and therefore they should be designed to allow for free right turns when possible. This
preference is reiterated in the California Highway Design Manual [3] where it states in
Index 504.3(2):
“Where a separate right turn lane is provided at ramp terminals, the turn lane should not
continue as a free right unless pedestrian volumes are low, the right turn lane continues as
a separate full width lane for at least 60 m [200 ft] prior to merging, and access control is
maintained for at least 60 m [200 ft] past the ramp intersection. Provision of the free
right should also be precluded if left turn movements of any kind are allowed within 125
m [410 ft] of the ramp intersection.”
Despite this foundation of design philosophy, the Guidelines also mentions that “often
free right turn moves at exit ramps can not be provided due to close proximity of adjacent
intersections.” (p.3) Close proximity of downstream intersections would not allow for
sufficient weave and merge lengths with a free right turn from the off-ramp.
A Policy on Geometric Design of Highways and Streets by AASHTO (the “Green Book”)
[4] provides further support for the use of free off-ramp right turns. On pages 748 and
787 the Green Book states “all right turns into and out of ramp approaches are generally
free flow…and only the left turns must pass through the signalized intersection.” The
Green Book also provides guidance on when free off-ramp right turns should be
implemented, “the design of the free right turns should include an additional lane on the
cross street beginning at the free right-turn lane for at least 60 m [200 ft] before being
merged. Free-flow right turns from the exit ramp to an arterial cross road are not
desirable when the nearest intersection on the cross road is within 150 m [500 ft] because
there may be inadequate weaving distance between the exit ramp and the adjacent
intersection.” The California Guidelines [2] criteria are quite similar with the additional
criterion of access control being maintained for at least 200 feet beyond the ramp
intersection. The Green Book still accounts for the possibility of the off-ramp right turn
being a controlled movement despite the details pertaining to free right turn situations.
The Minnesota Department of Transportation Roadway Design Manual [5] also is a
proponent of free off-ramp right turn movements. It states that “left and right turn
movements at single point diamond interchanges (SPDI) should be physically separated,
and moreover allow the right turns to flow independent of the signal.” (p. 6-1(3)) The
basis for this statement is that any portion of the signal cycle length devoted to the off-ramp
right turn movement increases the overall interchange delay.
8
Figure 1. Common Right Turn Lane Configurations at Exit Ramps
(California Single Point Interchange Planning, Design, and Operations Guidelines [2])
9
NCHRP 345 [1] states that “in general, the right-turn maneuver will operate more safely
and efficiently if a right-turn bay and auxiliary lane are provided” (p. 24) because the
traffic flows are physically separated. However, the design guidelines presented in the
report state that “an acceleration lane for off-ramp traffic onto the cross arterial is not
necessarily recommended unless sufficient distance (greater than 1,200 feet) is available
to the next downstream [signalized] intersection. Direct entry merging for this maneuver
provides good operation in restricted designs.” (p. 99)
Controlled Off-Ramp Right Turns
Despite the emphasis placed on free off-ramp right turns by the preceding sources, the
same sources as well as others provide some detail pertaining to controlled off-ramp right
turns. Primarily, the controlled movement aspect is concerned with signalization,
although some discussion is provided as it relates to yield and stop control types.
Signal Control
The California Guidelines [2] qualify its preference for free right turns with the provision
that when volumes are too high for one exit ramp right turn lane it is sometimes
reasonable to add and signalize another exit ramp lane exclusively for right turn
movements. This situation, as well as other approaches to off-ramp right turn movement
control in California, is shown in Figure 1 as Item 2-300(2).
The Guidelines also contends “in some situations this configuration of a combination free
right/signalized right turn layout can mitigate short weaves and merges related to close
spacing of the ramp and adjacent local intersections.” (p. 10) According to the Guide-lines,
signalization of the off-ramp right turn is considered when the spacing between the
ramp and the adjacent intersection is too short and/or there is a large proportion of right
turn traffic from the exit ramp attempting to weave across the cross street to turn left at
the adjacent intersection. This situation is depicted in Figure 1 as Item 2-300(3).
Page 113 of the NCHRP Report 420: Impacts of Access Management Techniques by
Gluck, et al [6] notes that signalization of the off-ramp right turn can be used to alleviate
(to some degree and dependent on progression considerations) congestion at downstream
signals sometimes caused by free or yield-controlled off-ramp right turns. The
signalization of the off-ramp right turns also can assist motorists with shorter
weave/merge lengths or to accommodate a heavy left-turn demand at the downstream
location. The report also cautions that the signalization of the off-ramp right turns may
cause an increase in the queue length, which must be minimized to avoid spillback onto
the freeway mainline. The AASHTO Green Book [4] also provides this same advice, but
applies it to possibly blocking access to the off-ramp left turn lanes (or through
movement if the SPUI has frontage roads). The Utah Department of Transportation
(UDOT) currently has a project in design at this time to signalize most of these off-ramp
right turns in Salt Lake County [7]. UDOT cites problems with traffic queues extending
back onto the mainlines. They feel that replacing their current stop controls (they do not
have yield control) with signal control will allow for traffic to still turn right after
stopping when the signal is red, but will more importantly “flush out” the traffic queue
via a green signal indication when no conflicting movements are operating.
When signal control is utilized, the California Guidelines [2] states that right turns on red
should be allowed when practical and should have a sign stating they are allowed or not
allowed. According to the Guidelines, the use of the sign “will reduce the risk of
driver confusion on the nature of this movement and in enforcement.” The typical location of
the off-ramp right turn movement signals is shown in Figure 2 (Note: “OLA” refers to
the phasing being an overlap of the corresponding cross street left turn phase). The
Guidelines also points out that U-turns from the cross street are not allowed in this
situation since any U-turns would conflict with the off-ramp right turn movement
phase that is overlapped with the cross street left turn movement.
Figure 2. Signal Head Placement for Exit Ramp Right Turns
(California Single Point Interchange Planning, Design, and Operations Guidelines [2])
It is interesting to note that the California Guidelines, as shown in Figure 2, depicts the
signalization of the off-ramp right turn movement as a separate lane group apart from the
“free” off-ramp right turn movement. One possible reason for this design relates to
promoting an efficient operation of the signalized movement and safety of the vehicles
involved. By having the signalized off-ramp right turn movement intersect the cross street
10
11
at a right angle, the sight distance is not affected by the curvature of the typical off-ramp
right turn lane layout. The Guidelines suggests that interchange/off-ramp right turn
operations are affected by inadequate sight distance because “if drivers in a queue cannot
see approaching vehicles, each driver may tend to slow and creep into the intersection, thus
reducing the capacity of the ramp and hindering the operation of the intersection.” (p. 8)
NCHRP 345 [1] notes that signalized off-ramp right turn movements tend to work quite
efficiently during their green phase, but revert to stop-and-go situations for the red phase.
During this portion of the signal cycle, the flow rate for the operation is much lower,
which highlights the driver’s need to verify safe gaps to enter the cross street. Based on
observations presented in the report, off-ramp right turns controlled by signals “appeared
to operate about as efficiently as yield control.” (p. 27) The off-setting efficiencies of the
movement during the green and red phases were cited as the reason.
There is a method of addressing insufficient off-ramp right turn capacity without
resorting to signalization as detailed in NCHRP 345 [1]. Since the off-ramp right turn
movement does not have a “parent” phase to provide a protected entry, sometimes the
off-ramp right turns will not have adequate yield-entry merging capacity during high-volume
conditions. Usually this will only occur at SPUI sites with only one lane devoted
to the off-ramp right turn movement. The report describes the use of a queue detector,
located in the off-ramp right turn lane with yield control that is connected to the adjacent
(i.e., overlapping) cross street left turn phase:
“This delayed-call queue detector should be located perhaps 50 feet upstream from the
stop line (to detect the presence of the second or third vehicle stopped in queue). A
delayed call of perhaps 6 seconds would be adequate for a normal 6-foot by 6-foot
inductive loop detector design. If the queue remains over the loop for 6 seconds or more
during the cross street left turn red, a call is placed for the left turn phase to provide
‘protected’ right turns. If the left turn phase is already green, the ‘delay inhibit’ or defeat
feature of the detector-controlled system should be used to turn off the delay feature
during green, so that the right turn calls are immediately recognized to extend the cross-street
left turn phase until gap out. These features will provide additional movement
capacity only when needed by just monitoring the queuing status of the right turn. Single
vehicles stopping in line to make a right turn will still enter under yield control.” (p. 70)
The Design Guidelines presented in NCHRP 345 [1] state that “signalizing the off-ramp
right turn operations should be avoided. Delayed-call right turn queue detection should
be provided for high-volume conditions having fairly balanced traffic patterns. Right
turn volumes from the off-ramp exceeding the complementary cross street volume by 100
vehicles per hour per lane, vphpl, should warrant this detector treatment when the right
turn volume exceeds 300 vphpl.” (p. 99)
12
Yield Control
NCHRP 345 [1] provides many of the details pertaining to yield controlled off-ramp right
turn movements. It states that yield control “has the advantages of being relatively
efficient in terms of traffic performance and right-of-way need.” (p. 26) The main reason
for its efficiency is because it only requires the off-ramp turn traffic to stop when it
cannot safely enter the cross street traffic stream. Therefore, the movement is able to
make maximum use of opportunities to enter with a minimum amount of delay. The
capacity of an off-ramp right turn movement under yield control is highly sensitive to the
amount of conflicting traffic. Later in the report, the following statement is made,
“observation…suggests that yield control for the off-ramp right turn movement can be an
efficient and cost-effective control mode.” (p. 27)
SAFETY ASPECTS
The method of controlling the off-ramp right turn movement at SPUIs can also affect the
safety of the interchange. Several sources offered information supporting certain types of
off-ramp right turn control from a safety perspective. The safety concern highlighted in
the literature usually is associated with pedestrians and bicycles, but the type of off-ramp
right turn control can also affect vehicular safety.
PEDESTRIAN & BICYCLIST SAFETY
Pedestrians and/or bicyclists attempting to cross the off-ramp approach of a SPUI are
faced with unique conditions which warrant particular attention to ensuring that there is a
mutual understanding of the traffic situations by both the driver and the pedestrian/
bicyclist. The off-ramp right turn movement is of particular concern due to this being
one of the first points of potential conflict at the interchange.
The AASHTO Green Book [4] points out that heavy pedestrian traffic can diminish the
desirability of free right-turn lanes by adding a potential conflict with non-controlled
vehicular traffic. This situation is of particular concern when the off-ramp right turn
lane(s) are curved in such a way as to promote a speed sufficient for merging with the
cross street and yet obscure the intervisibility between the driver and pedestrian. NCHRP
Synthesis of Highway Practice 139 [8], which provides general information regarding
expressway ramps intersecting local streets, states that, “…vehicles are still traveling at a
relatively high rate of speed when they pass through the intersection or merge with
surface street traffic.” (p. 38) The report continues by indicating motorists also may be
unaware of pedestrians because they are focused on looking for upstream traffic. This
behavior would probably be evident regardless of the traffic control in place since the
driver is either anticipating a gap for a right turn on red (or at a stop control) or timing a
gap for a yield or free right turn/merge situation. Based on this situation, NCHRP 139 [8]
also states that “…pedestrian safety can be severely threatened at intersections where
freeway off-ramps intersect with local streets, because of the high-speed traffic mixing
with crossing pedestrians.” (p. 39)
13
The report followed up on this idea with the following:
Situations where high-speed expressway ramps intersect with local streets were identified
as having lessened adverse effects when:
􀂃􈌠 pedestrian volumes and local traffic volumes are relatively low and good roadway
designs are used
􀂃􈌠 suitable traffic control devices are used at the local street and/or grade separation
(where appropriate)
The conditions listed as possibly harmful include:
􀂃􈌠 High traffic volumes and/or speeds on the off-ramp
􀂃􈌠 Moderate to high pedestrian volumes crossing at the intersection
􀂃􈌠 Insufficient traffic controls at the intersection (e.g., off-ramp traffic controlled by yield
signs only)
􀂃􈌠 High-speed traffic on ramp having poor sight distance and/or an unexpected
intersection
The conclusions drawn from the report suggest that the hazards to pedestrians can be
mitigated by using proper intersection design, utilizing grade separation, and/or
implementing adequate traffic control devices (e.g., signals and signs). The effects of
these items are reductions in vehicle speeds and increased pedestrian/motorist awareness.
NCHRP 345 [1] suggests that signalizing the off-ramp right turn movements would
reduce the capacity of the SPUI as a whole. Also, the capacity of the off-ramp right turn
movement would be similar to that of a yield-controlled movement because the increased
efficiency of operation during the green phase is partially offset by the reduced efficiency
during the red phase. Furthermore, the report mentions observations from its associated
field study which showed “pedestrian behavior…indicated that pedestrians were able to
cross the ramp junctions safely and with little confusion as to when it was safe to cross
during the cycle.” (p. 32)
The California Guidelines [2] had some limited safety information concerning bicyclists.
The Guidelines promotes only one lane being dedicated as a free right turn from the exit
ramp “so bicyclists need to cross only one lane of uncontrolled traffic.” (p. 11) Also, the
use of stop control for the exit ramp right turn traffic is mentioned as a means of
adequately accommodating bicyclists in some situations. Furthermore, the Guidelines
states that if an exit ramp right turn lane is anticipated to be signalized in the future or if
the SPUI is larger than a “compact” SPUI as defined by the Guidelines, then a separate
bicycle facility (i.e., overpass or underpass) should be incorporated into the SPUI design.
Vehicular Safety
None of the literature sources reviewed had specific information pertaining to the crashes
associated with the off-ramp right turn movement. Data and conclusions pertaining to the
off-ramp as a whole were evident. The Minnesota Department of Transportation
Roadway Design Manual [5] states “the predominant crash type at SPDIs [SPUIs] is rear-
14
end crashes on the off-ramp.” (p. 6-1(5)) This conclusion is further supported by the
Cheng article, “Accident Analysis for Single Point Urban Interchange” [9] which states
the predominant type of crash is rear-ends on the off ramps with a reported percentage of
at least 40%. This paper advises that improvements in advance warning signs, visibility,
location of signal and stop bar, and skid resistance could reduce off-ramp rear-end
crashes.
The radius of the off-ramp right turn lanes also contributes to the safety of the movement.
NCHRP 345 [1] found that almost all stop, yield, and traffic-signal controlled off-ramp
right turn movements had radii of less than 100 feet. Radii of this size or smaller
promote better visibility for off-ramp right turn motorists as they look back to their left to
assess cross street traffic conditions. However, the assessment of potential vehicular
traffic conflicts complicates any off-ramp right turn movement regardless of turn radius
or traffic control (except possibly free right/merge). The report emphasizes this with the
following statement, “…the greater distance and unique phasing create a complex flow
pattern by releasing a second platoon a few seconds after the through phase. This second
platoon may surprise right turning drivers who expect to enter freely after the end of the
cross-road through phase.” (p. 27) The origin and sequencing of the conflicting traffic
streams is not consistent with the expectancy of a driver making the off-ramp right turn
maneuver. This could be the basis for the right-angle and rear-end collisions associated
with the off-ramp right turn. One form of mitigation would be to separate the entry point
farther from the interchange via a merge control. Usually, this is not feasible due to
space constraints and/or the proximity of a downstream signalized intersection where left
turns are permitted.
The California Guidelines [2] focuses on safety by describing desirable visibility
conditions. The Guidelines promotes intervisibility and claims that this will improve
safety conditions and operational conditions.
CONCLUSIONS
The literature and research documented above show that there is some attention devoted
to the operation and safety of SPUIs specifically pertaining to the off-ramp right turn
movement. The literature also revealed that there does not appear to be any past or
present research/studies investigating the advantages and disadvantages of using one
form of control over another for the off-ramp right turn movement.
With regards to operational/design effects, this research paper should focus on several
key points. The intersection downstream of the off-ramp right turn movement is
important to the selection of the traffic control used at the off-ramp right turn. The
information reviewed showed that free right turns are a common practice, but are
constrained by the downstream intersection location. Signal control at the off-ramp right
turn can “meter” the off-ramp right turn traffic and help with shorter weaving distances
and congestion at the downstream intersection. NCHRP 345 [1] promotes a distance
between the SPUI and the downstream intersection that provides enough room to store
stopped cross street traffic as well as provide additional room to accommodate lane
15
changes/weaving in advance of the stopped cross street traffic. The report recommends a
desirable downstream signalized intersection separation of at least 1,200 feet from the
off-ramp entry point. Spillback from a close downstream signalized intersection can
affect the efficiency and safety of the off-ramp right turn movement.
The information reviewed described situations where pedestrian, bicyclist, and motorist
safety can be affected by the type of control used for the off-ramp right turn movement.
Other factors such as geometric design, sight conditions, pedestrian/bicyclist activity, and
vehicle speeds also play significant roles, but the traffic conditions in which these all
interact can be exacerbated or enhanced from a safety perspective based on the control
type in place for the off-ramp right turn movement.
The review of information also indicated some concepts that will assist in the evaluation
tasks of this project. Most of the information from the research papers by Follmer and
Janson [10] and Bonneson [11] concern the evaluation of signal operations at SPUIs. For
instance, the Follmer/Janson paper proposes an alternative to using the simple Highway
Capacity Manual (HCM) estimate for right turn on red (RTOR) capacities. The concept
is that a motorist attempting to turn right on red at a signalized intersection from an
exclusive right turn lane will encounter similar conflicting traffic flows to a motorist
attempting to turn right at an unsignalized intersection.
Another concept related to SPUIs with signalized off-ramp right turn movements is
clearance time. The Bonneson paper [11] defines the clearance interval as the “interval
[that] follows the yellow warning interval at the end of each signal phase. It is intended
to provide sufficient time for those vehicles entering during the yellow to safely clear the
intersection conflict area before the start of the next phase.” (p. 11) When the off-ramp
right turn movement is signalized, essentially the interchange has “grown” to incorporate
a larger conflict area. Thus, the clearance time has to be longer based on this increased
potential conflict area. As Bonneson [8] puts it, “Longer clearance intervals lead to
longer delays for the motorist because all-red time represents time that is not available to
serve traffic demand.” (p. 6) This important point is emphasized in NCHRP 345 [1]
which claims this situation as a “major disadvantage of signal control for the off-ramp
right-turn movement.” (p. 27) However, this facet of SPUI operation does not preclude
the use of a signalized off-ramp right turn movement; it merely means the “…designer’s
goals should be to minimize the length of the clearance paths while still providing a
geometric design that meets or exceeds minimum design standards.” [11]
16
17
CHAPTER 2
EXISTING CONDITIONS AT STUDY SITES:
OFF-RAMP RIGHT TURN CONTROL AT
SINGLE POINT URBAN INTERCHANGES (SPUI) WITHOUT
FRONTAGE ROADS
INTRODUCTION
This study is concerned with the evaluation of off-ramp right turn control options at
single point urban interchanges (SPUIs) without frontage roads. The off-ramp right turn
control employed at a SPUI can affect the interchange as a whole as well as the specific
off-ramp right turn movement. This study is concerned with operational efficiency (also
referred to as “operations” within this report) and safety. The analysis of each
component will be compared on equal terms in order to determine the advantages and
disadvantages of certain off-ramp right turn traffic controls. The first step in this
evaluation process is the data collection effort.
Ideally traffic data relating to volumes, operations, and safety would be readily at-hand
for any number of subject sites. Without this luxury, some concessions had to be made in
order to conduct this study. The number of interchange sites to be studied was limited by
the funding available with acknowledgment that the more sites that were studied, the
more useful and applicable the information would be. In order to supplement this
constraint, the data collection effort was geared towards providing information that could
be used to calibrate a micro-simulation model (CORSIM) that could then be used to
evaluate a myriad of hypothetical SPUIs with varied traffic volumes/distributions and
off-ramp traffic controls. Although these interchanges technically would not exist, their
operation and subsequent evaluation would be a derivative of actual data collected as
described in this chapter
DATA COLLECTION EFFORT
The data collection activities were related to the two main aspects being evaluated in this
study: off-ramp right turn operations and safety. All operational data were collected in-field
over the course of several weeks in early 2004. Some of the in-field safety data
were obtained through engineers’ observations and recordings, but a significant portion
of the safety-related data was from historical crash records. The following subsections
describe the data collection process while subsequent sections report findings and
calculations based on the data obtained.
Study Site Selection
Six SPUI sites were selected for study in this research project. There were several
criteria that controlled which sites would be viable. First and foremost, the SPUI had to
be a “three-phase” (referring to the signal phasing necessary) configuration meaning it
did not have frontage roads incorporated into its operation. The second criterion was that
18
Figure 3. Study Sites
Not to Scale
51
51
101
202
101
202
Note: The pushpins indicate the study sites.
19
the SPUI had to have sufficient crash history data available, i.e., be fully operational for
at least three years. Applying these two criteria resulted in 17 potential sites in the
Phoenix metropolitan area. The next level of filtering was based on the type of off-ramp
right turn control used at the potential sites. Five of the seventeen sites had signalized
off-ramp right turn controls, the remainder used yield control for the off-ramp right turn
movement. The final selection of the six study sites was determined by the technical
advisory committee (TAC) which relied on lane configuration information,
pedestrian/bicycle activity, and local knowledge of the interchanges. The resulting study
sites listed below provide a mixture of operation types and configurations commonly
found in the Phoenix area:
􀂃􈌠 State Route 51 (SR 51) & Indian School Road
􀂃􈌠 State Route 51 (SR 51) & Glendale Avenue
􀂃􈌠 State Route 51 (SR 51) & Cactus Road
􀂃􈌠 State Route 51 (SR 51) & Greenway Road
􀂃􈌠 Loop 101 (Agua Fria Freeway) & Bell Road
􀂃􈌠 Loop 202 (Red Mountain Freeway) & Rural (Scottsdale) Road
Figure 3 shows the general location of the interchanges while Figures 4 through 9 are
aerial photographs of each interchange. Table 1 shows the pertinent characteristic data
for each interchange.
There are some important aspects to keep in mind when reviewing data, analysis, and
findings concerning the selected study sites. Although a majority of the interchange sites
were oriented with the freeway aligned north-south, the Loop 202/Rural Road
interchange has the freeway aligned east-west. Also, the Loop 101/Bell Road and SR
51/Greenway Road interchanges have a skewed configuration, although the freeway
generally aligns north-south. The freeway alignment could potentially affect driver
vision caused by sun glare. Another difference between the interchanges that could
factor into inherent interchange characteristics is the method of separating the freeway
from the cross road. The interchange could be configured with the freeway passing over
the cross road (an overpass interchange) or the freeway passing under the cross road (an
underpass interchange). Either configuration may have advantages and disadvantages
relating to interchange operations and safety. Half of the study sites selected were of the
overpass interchange variety with two of these three sites also having signalized off-ramp
right turn movements. Yet another variation was present at the SR 51/Glendale Avenue
interchange where the northbound off-ramp right turn movement was controlled by a
traffic signal and the southbound off-ramp right turn movement was yield-controlled.
This mixture of off-ramp right turn traffic controls prompts particular attention to the
analysis of the overall interchange operation while also providing a microcosm to
potentially compare the two methods of control. Ideally the study site selection would
have attempted to minimize, if not eliminate, these characteristic variables through
consistency, but given the availability of potential study sites meeting the primary
selection criteria stated previously, this was not possible.
20
Figure 4. State Route 51 & Indian School Road Aerial Photograph
Photo source: Maricopa County Assessor’s Office
Photo date: December 2002
SR 51
Indian School Rd
1’ = 100’
21
Figure 5. State Route 51 & Glendale Avenue Aerial Photograph
Photo source: Maricopa County Assessor’s Office
Photo date: December 2002
SR 51
Glendale Ave
1’ = 100’
22
Figure 6. State Route 51 & Cactus Road Aerial Photograph
Photo source: Maricopa County Assessor’s Office
Photo date: December 2002
SR 51
Cactus Rd
1’ = 100’
23
Figure 7. State Route 51 & Greenway Road Aerial Photograph
Photo source: Maricopa County Assessor’s Office
Photo date: December 2002
SR 51
Greenway Rd
1’ = 100’
24
Figure 8. Loop 101 (Agua Fria Freeway) & Bell Road Aerial Photograph
Photo source: Maricopa County Assessor’s Office
Photo date: December 2002
Loop 101
Bell Rd
1’ = 100’
25
Figure 9. Loop 202 (Red Mountain Freeway) & Rural Road Aerial Photograph
Photo source: Maricopa County Assessor’s Office
Photo date: December 2002
Rural Rd
Loop 202
1’ = 100’
26
27
Operations-Related Data
The data on interchange/off-ramp right turn operations has three elements: traffic
volume, interchange signal timing/phasing, and off-ramp right turn specific delays. The
procedures used to collect data on each of these elements are described below and the
resulting information presented accordingly.
Traffic Volumes
Data relating to traffic volumes was fundamental to the evaluation of the study sites.
Two-way daily traffic volumes were collected at each interchange with a majority of the
other data collection efforts occurring simultaneously. The daily traffic volumes were
collected using automatic traffic recorders (ATRs) which consist of a counter and
pneumatic tube placed at selective locations within the interchange area. Specific
volumes for the movements through the interchange were also recorded by data collectors
for a one and a half hour period in the morning and evening. The resulting volumes are
shown in Figures 10 through 15.
The number of right turns made on red from the off-ramp was recorded. Additionally,
the number of heavy trucks was noted and used to calculate truck percentages for the
interchange. The raw data from the turning movement and daily traffic collections are
contained in the Appendix A.
The traffic volume data was collected in January 2004. Review of Arizona Department
of Transportation (ADOT) seasonal adjustment factors revealed that January is one
percent higher than the annual average month for the Phoenix area. Therefore, the
volumes presented previously were adjusted downward by 1% prior to any computations
being performed.
Interchange Signal Timings
Even though this study is specifically focused on the operations and safety related to the
off-ramp right turn movements at SPUIs, the control employed at the off-ramp right turn
can have an effect on the overall interchange efficiency. To account for this, signal
timing information was required so that the entire interchange could be evaluated from an
operations standpoint.
The overall interchange signal timing/phasing and the specific timing/phasing associated
with the off-ramp right turn movement were collected from the governing agencies.
Actual signal timing samples were recorded in the field in order to verify, to a certain
degree, the information provided by the agencies. Generally, the in-field timing samples
concurred with supplied timing information which was then used for calculations relating
to delay and overall interchange operations via the CORSIM modeling.
28
29
30
31
32
33
34
Off-Ramp Right Turn Delay
Particular attention was devoted to the off-ramp right turn movement operations during
the data collection tasks. The primary indication of operational efficiency for the off-ramp
right turn movement is the delay incurred by the motorist due to the control device,
whether a signal or yield sign, and prevailing traffic conditions. In order to determine
this average delay per vehicle, a data collection procedure from the Highway Capacity
Manual (HCM) [12] was used as a guide. The procedure is primarily dependent on three
components of traffic data: volume over a specified period of time, number of vehicles
stopping during that time, and number of vehicles considered part of a queue in the off-ramp
right turn traffic flow. The data collected was for the one and one-half hour peak
periods in the morning and evening.
The traffic volume for the off-ramp right turn movement was collected in conjunction
with the turning movements for the entire site. The number of off-ramp right turn vehi-cles
that were counted as a vehicle that stopped was based on observing a vehicle come to
a full stop at any point along the length of the off-ramp right turn lane(s) up to and
including the junction point with the cross road. If the same vehicle stopped multiple
times, it was only recorded as one stopped vehicle in the count total. Vehicles counted as
being part of a traffic queue constituted any vehicle within one vehicle length of another
vehicle, whether one or both vehicles were moving or stopped. Additionally, one off-ramp
right turn vehicle waiting to turn right onto the cross road was considered a queue
of one. These vehicle-in-queue determinations were assessed every 19 seconds per the
HCM data collection guidelines, which require the interval to be any value up to 20
seconds so long as the interval does not divide evenly into the cycle length for the inter-change.
Nineteen seconds was selected as the observation interval because this interval
value would not divide evenly into any of the signal cycle lengths used at the study sites.
Observing/recording the traffic queues in this manner results in a random sample of
values, which were then used in the calculation of the control delay for the movement.
Safety-Related Data
The analysis of the safety implications related to the off-ramp right turn control type was
supported by data collected concerning conflicts observed and crash history investigations.
Conflict observations were conducted by experienced traffic engineers, one positioned at
each off-ramp right turn area, during the AM and PM peak periods. Crash histories were
obtained for each interchange that has the off-ramp right movement only. Each of these
data sets were then used in conjunction with the traffic volume data to determine both
conflict and crash rates specifically related to the off-ramp right turn movement.
Conflict Observations
Although traffic crash records provide the most direct measure of safety for a roadway
section, adequate data may not be available for analysis. Moreover, some crashes are not
reported or records may be only available for a time period that does not represent current
conditions at the study area. Therefore, conflict data specifically pertaining to the off-ramp
right turn movements was obtained for the AM and PM peak periods at the study sites.
35
For the purposes of this study, a conflict was considered to be a traffic event involving
two or more road users (i.e., vehicles, pedestrians, bicyclists, etc.), in which one or more
user performs an abnormal or unusual action causing another or others to execute an
abrupt or evasive maneuver to avoid a collision. The most common avoidance maneuver
related to the off-ramp right turn movement is either abrupt braking or swerving to avoid
a collision.
The decision concerning what traffic occurrence/situation constitutes a traffic conflict is
subjective to some degree. In an attempt to minimize observer subjectivity, only
experienced engineers conducted the conflict observations. The same two engineers were
used at every study site location. The observation positions were chosen on a site-by-site
basis based on whichever position provided the best vantage point to observe conflicts
involving off-ramp right turn traffic interacting along the off-ramp or at the ramp junction
with the cross road where the cross road traffic could also be involved. The following
guidelines were used in identifying traffic conflicts:
􀂃􈌠 Secondary conflicts caused by an initial or primary conflict were possible at
the study sites. If this occurred, a maximum of one secondary conflict was
recorded and tabulated as a separate traffic conflict.
􀂃􈌠 Unusual occurrences due to the presence of ambulances, fire trucks, or police
vehicles were identified but not included in the conflict observation tally.
􀂃􈌠 Example of non-conflict occurrence: a driver performing normal braking due
to the presence of a yellow/red signal or resulting traffic queue.
􀂃􈌠 Example of a conflict occurrence: a driver who brakes abruptly to avoid a
collision with a vehicle slowing for a yellow/red signal because they
anticipated following the vehicle through the signal.
In order to assist with the observation and recording of traffic conflicts, a schematic key
map was developed to identify the location of conflicts. The key map is shown in Figure
16 below. The numbered location areas are intended to be general in nature and to cover
all areas of potential conflicts, although observations found that most conflicts were
confined to one or two main areas. This same key map was also used for the crash
history investigations.
When conflicts were observed, four items of information were recorded: the time, the
location (per the key map), the types of road users and their associated movement, the
avoidance actions taken, and a more detailed account (if necessary). Observed conflicts
were recorded on standardized sheets used at each study site.
The data collected from the conflict observations will be presented in the Calculations
section of this chapter in conjunction with the calculated conflict rate values.
Crash History Investigation
The crash history investigations used data from ADOT’s Accident Location Identification
Surveillance System (ALISS) database, which was queried for the most recent three-year
36
period of crash information (August 1, 2000 through July 31, 2003) at the time of the
request. The query consisted of any crashes occurring specifically in the right turn
lane(s) on the off-ramp or at the crossroad. Crashes reported as occurring on the cross
road involving an off-ramp right turn vehicle were also included in the query request.
The effective distance for the query was set at 300 feet from the off-ramp right turn/cross
road junction point. The resulting number of crash records returned from the query was
about 650 for the six interchanges (twelve off-ramps) for the three-year period.
The listing of crash records was then used to retrieve the actual crash reports from
ADOT’s Traffic Records Section. The actual crash reports were reviewed by traffic
engineers to determine their applicability to the off-ramp right turn movement. Overall,
only a small percentage (~2%) were found to be inapplicable and were thus removed
from the crash record listing for the respective study site location. During the review of
the crash reports, the location of the crash was noted according to the key map shown as
Figure 16. This determination was somewhat subjective since the crash reports usually
provided a sketch of the crash location relative to geometric aspects of the interchange.
Generally, Area 3 was reserved for crashes occurring within one to two vehicle lengths of
the junction point with the cross road in addition to crashes involving cross road traffic in
the curb lane. Area 4 was reserved for other crashes occurring farther away, bounded by
the gore point on the off-ramp. Tables 2-13 present the crash data totals.
Figure 16. Schematic Conflict/Crash Location Key Map
37
Signal Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 45 8 2 0 0 0
Rear-End 45 8 2 0 0 0
Location 1
Location 2
Location 3 45 8 2
Location 4
Table 2. SR 51/Indian School Road - Southbound Off-Ramp Right Turn Related Crashes
*
Signal Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 21 8 0 0 0 0
Rear-End 21 8 0 0 0 0
Location 1
Location 2
Location 3 21 8
Location 4
Table 3. SR 51/Indian School Road - Northbound Off-Ramp Right Turn Related Crashes
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 55 11 3 0 0 0
Rear-End 51 11 2 0 0 0
Location 1
Location 2
Location 3 51 10 1
Location 4 1 1
Sideswipe (same dir.) 1 0 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Single Vehicle 2 0 0 0 0 0
Location 1
Location 2
Location 3
Location 4 2
Angle 1 0 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Pedestrian-Involved 0 0 1 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Table 4. SR 51/Glendale Avenue - Southbound Off-Ramp Right Turn Related Crashes
Locations refer to Figure 16
*
*
*
* Locations refer to Figure 16
* Locations refer to Figure 16
38
Signal Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 9 1 0 0 0 0
Rear-End 9 1 0 0 0 0
Location 1
Location 2
Location 3 8
Location 4 1
Location 6 1
Table 5. SR 51/Glendale Avenue - Northbound Off-Ramp Right Turn Related Crashes
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 8 2 0 0 0 0
Rear-End 8 2 0 0 0 0
Location 1
Location 2
Location 3 8 2
Location 4
Table 6. SR 51/Cactus Road - Southbound Off-Ramp Right Turn Related Crashes
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 70 28 2 0 0 0
Rear-End 65 28 2 0 0 0
Location 1
Location 2
Location 3 57 24 2
Location 4 8 4
Sideswipe (same dir.) 2 0 0 0 0 0
Location 1
Location 2
Location 3 2
Location 4
Single Vehicle 2 0 0 0 0 0
Location 1
Location 2
Location 3 2
Location 4
Backing 1 0 0 0 0 0
Location 1
Location 2
Location 3
Location 4 1
Table 7. SR 51/Cactus Road - Northbound Off-Ramp Right Turn Related Crashes
*
* Locations refer to Figure 16
* Locations refer to Figure 16
* Locations refer to Figure 16
*
*
39
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 3 0 0 0 0 0
Rear-End 3 0 0 0 0 0
Location 1
Location 2
Location 3 3
Location 4
Table 8. SR 51/Greenway Road - Southbound Off-Ramp Right Turn Related Crashes
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 46 17 2 0 0 0
Rear-End 46 17 2 0 0 0
Location 1
Location 2
Location 3 41 16 1
Location 4 5 1 1
Table 9. SR 51/Greenway Road - Northbound Off-Ramp Right Turn Related Crashes
Signal Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 34 7 3 0 0 0
Rear-End 30 6 3 0 0 0
Location 1
Location 2
Location 3 27 5 3
Location 4 3 1
Sideswipe (same dir.) 3 0 0 0 0 0
Location 1
Location 2 2
Location 3 1
Location 4
Backing 1 0 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Pedestrian-Involved 0 1 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Table 10. Loop 101/Bell Road - Southbound Off-Ramp Right Turn Related Crashes
*
*
*
* Locations refer to Figure 16
* Locations refer to Figure 16
* Locations refer to Figure 16
40
Signal Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 35 11 5 0 0 0
Rear-End 33 11 4 0 0 0
Location 1
Location 2
Location 3 27 9 4
Location 4 6 2
Sideswipe (same dir.) 1 0 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Single Vehicle 0 0 0 0 0 0
Location 1
Location 2
Location 3
Location 4
Backing 1 0 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Bicyclist-Involved 0 0 1 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Table 11. Loop 101/Bell Road - Northbound Off-Ramp Right Turn Related Crashes
*
* Locations refer to Figure 16
41
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 75 19 2 2 0 1
Rear-End 70 19 2 2 0 0
Location 1 1
Location 2
Location 3 67 18 2 2
Location 4 3
Sideswipe (same dir.) 2 0 0 0 0 0
Location 1
Location 2
Location 3 2
Location 4
Single Vehicle 2 0 0 0 0 1
Location 1
Location 2
Location 3 2
Location 4 1
Angle 1 0 0 0 0 0
Location 1
Location 2
Location 3 1
Location 4
Table 12. Loop 202/Rural Road - Westbound Off-Ramp Right Turn Related Crashes
*
* Locations refer to Figure 16
42
Yield Controlled
(08/01/00 - 07/31/03) No Injury
Possible
Injury
Non-
Incapacitating
Injury
Incapacitating
Injury Fatality Unknown
TOTAL 79 23 4 2 1 0
Rear-End 70 22 4 2 0 0
Location 1
Location 2
Location 3 68 20 4 2
Location 4 2 2
Sideswipe (same dir.) 6 0 0 0 0 0
Location 1
Location 2
Location 3 5
Location 4 1
Single Vehicle 2 0 0 0 0 0
Location 1
Location 2
Location 3
Location 4
Location 8/9 2
Angle 1 1 0 0 0 0
Location 1
Location 2
Location 3 1 1
Location 4
Bicyclist-Involved 0 0 0 0 1 0
Location 1
Location 2
Location 3 1
Location 4
Table 13. Loop 202/Rural Road - Eastbound Off-Ramp Right Turn Related Crashes
If a particular type of crash was not listed in the above tables, then no crashes of that type
were found to have occurred in the three-year assessment period. The crash types listed
are based on the information noted by the officer on the actual crash report. The above
data was used in conjunction with the volume data (or derivatives thereof) to calculate the
crash rates for the off-ramp right turn movements at the study sites. These calculations
along with other calculations pertaining to the data described and presented previously
are explained and contained in the following section.
CALCULATIONS
This section presents the calculations performed using the operational and safety data.
The operational data collected specifically for the off-ramp right turn movement is used
to calculate the control delay (i.e., the portion of overall delay that results when a vehicle
slows or stops due to the presence of a traffic control like a signal or yield sign) for the
movement in the AM/PM peak periods/hours. Conflict observations are used with
volume data to determine conflict rates for existing conditions. The volume data and past
projections of volumes are also used with the crash history data to determine off-ramp
right turn movement crash rates for the three-year assessment period.
*
* Locations refer to Figure 16
43
Control Delay Calculations
The calculation of the control delay for the off-ramp right turn movement is fairly
complicated, relying on several factors and values supplied by tables in the HCM [12].
The general description of the calculation is shown below with the detailed description
and an example provided in the Appendix B. The main components which are used to
calculate the average control delay value per vehicle are:
(1) Time-in-Queue per Vehicle (seconds) =
(Count Interval [19 seconds] * (Sum of Vehicles Observed in Queue /
Total Off-Ramp Right Turn Volume)) * 0.9 [HCM correction factor]
(2) Number of Vehicles Stopping Per Lane Per Cycle Length (vehicles) =
Number of Vehicles that Stopped One or More Times / (Number of Signal
Cycles Observed * Number of Off-Ramp Right Turn Lanes)
(3) Acceleration/Deceleration Correction Delay Value (seconds) =
Ratio of Off-Ramp Right Turn Vehicles That Stopped * Acceleration/
Deceleration Factor [from HCM table—either +2 or +5 in this study
based on the Equation 2 results and free-flow speed range estimate]
(4) Average Control Delay per Vehicle (seconds) =
Equation 1 + Equation 3
These calculation procedures were performed for each off-ramp right turn movement at
the study sites regardless of the traffic control in place. Even though there was not a
portion of the signal cycle length devoted to the off-ramp right turn movements where
yield control was in place, the cycle length value for the interchange was still assumed in
the control delay calculations. This assumption is based on the yield control operation
being a derivative of gap acceptance in the cross road traffic stream for off-ramp right
turn traffic. These gaps are created by the traffic pattern fluctuations and by the cycling
of the overall interchange signal control. Control delay calculations for off-ramp right
turn movements at signal and yield control sites are similar since most of the delay is
generated as a function of gap acceptance: right turn on red at the signal control sites and
yielding right-of-way at the yield control sites.
The calculated delay results are shown in Table 14 (p.44). Please note that calculations are
provided for the peak period and the peak hour. Since the data component pertaining to
number of vehicles that stopped one or more times was collected only for the peak period
(i.e., the 1 ½ hour observation period), the peak hour value was pro-rated based on the
proportion of time. Since the peak period and peak hour durations were relatively close,
this assumption should not have a significant effect on the peak hour delay calculations.
Other data collected and used in the control delay calculation was specified as to whether
it pertained to the peak period and peak hour.
44
45
Conflict Rate Calculations
The conflict rate for the off-ramp right turn movement is the ratio of the number of
conflicts occurring and the volume of traffic that could potentially be involved in the
conflicts. The volume component is comprised of the cross road traffic (both through
volume, and volume generated by the opposing off-ramp left turn movement) and the off-ramp
right turn traffic. The conflicts and volume are summed for the same period of time
and the resulting ratio is multiplied by 1,000 to equate the value of the rate to typical
crash rate values. The calculation is shown below:
(5) RTCV = (CO / TCV) * 1000
where:
RTCV = Rate per thousand conflicting vehicles
CO = Conflicts observed
TCV = Total potentially conflicting vehicle volume
Table 15 presents the conflict data collected, the calculated conflict rates, and details
concerning the locations of the conflicts observed.
Interchange/Off-Ramp
Off-Ramp Cross Rd
1 2 3 4
ped/
veh
bike/
veh
veh/
veh other
Indian School/SR51 SB Off-Ramp 1 1033 3607 0.216 0 0 1 0 0 1 0 0
Indian School/SR51 NB Off-Ramp 0 558 3161 0.000 0 0 0 0 0 0 0 0
Glendale Road/SR51 NB Off-Ramp 2 690 5687 0.314 1 1 0 0 0 0 2 0
Bell Road/L101(W) SB Off-Ramp 3 2926 4843 0.386 0 0 2 1 0 0 3 0
Bell Road/L101(W) NB Off-Ramp 4 1885 4181 0.659 0 0 4 0 1 0 3 0
All Signal Control Off-Ramps 10 7092 21479 0.350 1 1 7 1 1 1 8 0
Glendale Road/SR51 SB Off-Ramp 3 944 4802 0.522 0 0 3 0 0 0 3 0
Cactus Road/SR51 SB Off-Ramp 2 848 2724 0.560 0 0 2 0 0 0 2 0
Cactus Road/SR51 NB Off-Ramp 7 770 3140 1.790 3 0 4 0 0 0 7 0
Greenway Road/SR51 SB Off-Ramp 1 476 3241 0.269 0 0 1 0 0 0 1 0
Greenway Road/SR51 NB Off-Ramp 1 1027 3176 0.238 1 0 0 0 0 0 1 0
Rural Road/L202 WB Off-Ramp 3 658 2981 0.824 1 0 2 0 0 0 3 0
Rural Road/L202 EB Off-Ramp 7 1109 2612 1.881 3 0 4 0 0 0 7 0
All Yield Control Off-Ramps 24 5832 22676 0.842 8 0 16 0 0 0 24 0
* RTCV = rate per thousand conflicting vehicles
Off-Ramp
Right Turn
Signal
Control
Off-Ramp
Right Turn
Yield Control
by Conflicts Period Traffic location (see key map) Involving
Observed
AM & PM
Conflict
Rate
(RTCV*)
Separate conflict rates for the AM and PM periods were not calculated due to limited
sample size. Instead, the conflict observation totals were combined and applied against
the total volume exposure over that collective duration. The conflict rate for the group of
off-ramp right turn movements segregated by control type was based on the aggregate
values of conflicts and volume instead of a simple average of the conflict rate values for
each individual off-ramp right turn movement. By doing this, the average for the group
is not biased as much by the variability of the conflict rate values caused by the relatively
small sample sizes.
Table 15. Conflict Data and Rate Computations for Off-Ramp Right Turn Movements
46
47
Crash Rate Calculations
The crash rate computations are similar to the conflict rate calculations, but are based on
a more robust time and sample. One difference in the rate computations is that the
resulting ratio of crashes to exposed volume is multiplied by one million rather than one
thousand to account for the greater volume considered over the longer assessment period
(in this case three years). Therefore, the crash rate is based on one million “entering”
vehicles (MEV) with “entering” constituting off-ramp right turn traffic volumes and the
traffic volume on the cross road immediately in front of the off-ramp right turn junction
area. Table 16 presents the calculated crash rates and corresponding data summary.
In order to calculate specific yearly crash rates for each off-ramp right turn movement,
additional volume data was obtained. Historical average daily traffic (ADT) volumes
were researched from governing city and state resources. Usually data was available for
the cross road on both sides of the interchange. Occasionally ADT data would only be
available for the cross road on one side of the interchange. These data ranged in age from
one to five years. All study sites had data pertaining to multiple years and so the most
recent years were used to formulate an average growth (or decline) rate. The data
collected in-field as part of this project served as the most recent value in the
determination.
The calculated growth rates for the study sites ranged from about -3% to about 4% per
year. The growth rate was applied to both the off-ramp right turn volume and cross road
volume immediately in front of the off-ramp. This included actually increasing the
volumes when projecting past yearly volume totals if the growth rate was a negative
value. Representations of volumes for previous years were generated from applying the
growth or decline rates to the existing volume data collected in 2004.
CONCLUSIONS
The data collected and the details obtained from observations and research allowed for
the calculations to be made concerning operations and safety. The interpretation of that
data through the results of the calculations lends itself to determining interchange
characteristics that influence operations and/or safety. One of these characteristics is the
traffic control for the off-ramp right turn movement and is the focus of this study.
Therefore, all of the calculation results have presented values that were grouped by the
individual off-ramp traffic control device—either signal or yield. The presentation of the
information in this manner allows trends specifically related to the traffic control used to
surface. The following subsections provide interpretation of the previously presented
data and highlight any trends and perspectives.
General Operations
Qualitative observations of off-ramp right turn traffic operations were facilitated through
the collection of conflict data. Other opportunities to observe and assess traffic
operations were possible during the data collection effort for the off-ramp right turn
48
control delay study. The following list highlights some important points relating to either
operation or safety (or both) for the study sites as a group:
􀂃􈌠 Motorists disregard the requirement to fully stop at a red signal indication when a
signal control is used for the off-ramp right turn movement.
􀂃􈌠 Due to this motorist disregard, the only significant difference in the off-ramp right
turn operations between signal control with right-turn-on red and yield control
occurs during the limited portion of the overall interchange cycle length when the
off-ramp right turn signal has a green arrow indication.
􀂃􈌠 The advantage of the green arrow phase associated with a signal-controlled off-ramp
right turn movement was perceived to be minimal as compared to a yield-controlled
off-ramp right turn movement since a fair amount of motorists were
observed not paying attention to the green arrow indication either by 1) looking
upstream along the cross road (away from the signal indication) or 2)
stopping/slowing in advance of the cross road (in preparing to look upstream)
despite the green arrow indication.
􀂃􈌠 Motorists’ tendencies to look upstream along the cross road while advancing
towards or being at the junction area (for either signal or yield controlled off-ramp
right turn movements) causes hardship on pedestrians attempting to cross the off-ramp
right turn lane(s), particularly when crossing from the motorist’s right side.
This is especially evident at sites using dual off-ramp right turn lanes.
􀂃􈌠 Pedestrian signal indications can be hazardous when the WALK indication is given
to a pedestrian crossing the off-ramp right turn lane(s) from the right of the motorist
since off-ramp right turn vehicles are either attempting to turn right on red or yield
which is dependent on gaps in the cross road traffic flow. To assess these gaps, the
motorist must look in the opposite direction from the pedestrian. This is especially
evident at signalized off-ramp right turn locations where the WALK indication is
given as soon as the cross road traffic receives its green indication. The width of
the interchange coupled with start-up time losses for the cross road through traffic
results in the creation of a sufficient gap for off-ramp right turn traffic to enter the
cross road on red at the same time the pedestrian WALK indication is given.
􀂃􈌠 Generally, queue lengths for the off-ramp right turn and off-ramp left turn
movements were not long enough to block access to either movement’s lane(s). If
blockage occurred, it was usually the build-up of off-ramp left turn vehicles
blocking the off-ramp right-turn vehicles, which could then usually pass the queue
by using the paved shoulder area existing outside of the lane line.
􀂃􈌠 Heavy off-ramp right turn conditions, primarily at signalized off-ramp right turn
locations, would prompt frustrated motorists to try to take every opportunity to
enter the cross road by turning during the limited change interval duration between
interchange signal phases. This would occasionally lead to off-ramp turn vehicles
turning onto the cross road during the end of (or after) the change interval time and
narrowly in front of an advancing platoon of vehicles from the cross road through
movement or opposing off-ramp left turn movement.
􀂃􈌠 Some motorists showed the tendency to want to follow the actions of the vehicle
immediately in front of them which led to or had the potential to lead to the lag
vehicle entering the cross road during insufficient gaps and/or without looking
upstream along the cross road.
49
􀂃􈌠 Data showed that motorists tended to use the outside (curb) lane about twice as much
as the inside lane at the study sites that had dual off-ramp right turn lanes.
􀂃􈌠 There were some observations of motorists blatantly disregarding the red signal
indication at signalized off-ramp right turn locations when they approached the
junction area immediately after the yellow arrow phase. Perhaps these motorists were
taking advantage of the longer clearance interval at the interchange (as compared to a
typical intersection).
􀂃􈌠 Due to the approach angle of some off-ramp right turn lanes, vehicle deflection (and
subsequent speed reduction) were not as enhanced leading to motorist tendencies to
continue at their off-ramp speed rather than slowing down to assess the cross road
traffic conditions.
􀂃􈌠 Regularly, off-ramp right turn vehicle queuing would block pedestrian access to the
crosswalk across the off-ramp right turn lane(s). This is especially evident at sites
using dual off-ramp right turn lanes since the outside lane vehicle must pull closer to
the cross road in order to try to see around the off-ramp right turn vehicle occupying
the inside off-ramp right turn lane.
􀂃􈌠 U-turns from the cross road left turn lane could and did conflict with some off-ramp
right turn vehicles attempting to turn at the same time. Traffic signs explicitly
restricting U-turns from the cross road left turn lane were not observed at any of the
sites.
Control Delay
Review of the information and results shown in Table 14 yields some interesting
observations. The following are some of the key points derived from the review of the
information when considering the different off-ramp right turn traffic control types:
􀂃􈌠 No discernable trends of increased control delay per vehicle associated with a
particular peak time or particular direction when considering all sites.
􀂃􈌠 Average Control Delay per Vehicle for the AM & PM Peak Period (and Hour)
o Number of signalized off-ramp right turn movements with average delay of
30+ seconds: 2 off-ramps (2 off-ramps)
o Number of yield-control off-ramp right turn movements with average delay
of 30+ seconds: 0 off-ramps (0 off-ramps)
􀂃􈌠 Longest Control Delay per Vehicle by Control Type
o Northbound Off-Ramp Right Turn at SR 51/Glendale Avenue (signal
control)—AM Peak Period (and Hour): 51.57 seconds (53.44 sec.)
o Southbound Off-Ramp Right Turn at SR 51/Greenway Road (yield
control)—PM Peak Period (and Hour): 26.52 seconds (28.78 sec.)
􀂃􈌠 Non-weighted Traffic Control Group Averages of Control Delays for Combined
Peaks
o Signal Control – per Total Vehicles, Peak Periods (and Hours):
19.24 sec. (19.66 sec.)
o Signal Control – per Stopped Vehicles, Peak Period (and Hours):
29.42 sec. (30.75 sec.)
o Yield Control – per Total Vehicles, Peak Periods (and Hours):
14.38 sec. (15.00 sec.)
50
o Yield Control – per Stopped Vehicles, Peak Periods (and Hours):
23.87 sec. (25.74 sec.)
Conflict Rate Comparison
Conflict data was presented in Table 15 which also included the calculated rates. The
overall rates for the control type groups were based on a recalculation of the conflict rate
using the summed values for each sample site. An overall average of the crash rates
calculated for each site was not deemed appropriate given the variability inherent to
conflict observations based on the relatively short observation period as compared to
crash rate calculations. The following list remarks on the findings:
􀂃􈌠 Conflict rates for yield-controlled sites as a group are about 240% greater than the
overall rate for the signal-controlled group. However, a statistical t-test reveals
that this difference is not significant (tcalc = 1.705, t.05, v=10 = 1.812) because of the
variability of the conflict rates at the yield control sites and the small sample size.
􀂃􈌠 Thirty-two of the thirty-four total conflicts involved two or more vehicles while
the remaining two conflicts involved vehicles and bicycles/ pedestrians, which
were only observed at signalized off-ramp right turn sites (representing 20% of
the conflicts observed at signalized locations).
􀂃􈌠 Most conflicts occur in Area 3 (refer to Figure 16) regardless of the off-ramp right
turn control type. However one-third of the conflicts observed at yield-controlled
sites occurred in Area 1.
􀂃􈌠 The highest conflict rates (per thousand conflicting vehicles) calculated for the
individual sites were for the eastbound off-ramp right turn at Loop 202/Rural
Road (1.881) and the northbound off-ramp right turn at SR 51/Cactus Road
(1.790). These sites also had the largest number of occurrences outside of Area 3,
which were in Area 1.
Crash Rate Comparison
Crash data is more robust than the data used to calculate conflict rates. Therefore, the
results and conclusions drawn from the crash data should be more indicative of longer-term
trends and conditions at the site. The crash data and conflict should be used
simultaneously to draw conclusions concerning a particular site and what cause(s) might
be contributing to them. The crash information per site was presented in Tables 2
through 13 with a summarization and calculated crash rates shown in Table 16. Overall
crash rates for the control type groups were the averaged values of the three-year average
crash rate for each site in the group. The following conclusions were developed from the
review of this information:
􀂃􈌠 The average crash rate for yield-controlled sites as a group is almost double the
average crash rate for the signal-controlled sites. This ratio is comparable to the
conflict rate relationship between the two groups.
51
􀂃􈌠 A statistical t-test was performed on the average crash rate data for the yield-controlled
sites and the signal-controlled sites. All crash rates were considered,
which resulted in no significant difference in the average rates for each group.
􀂃􈌠 The two sites with the highest average crash rate over the three-year assessment
period are also the two sites with the highest conflict rates (eastbound off-ramp
right turn at Loop 202/Rural Road and northbound off-ramp right turn at SR
51/Cactus Road).
􀂃􈌠 The proportion of crashes occurring in Area 3 support the conflict observations
that showed this being the most prevalent location for conflict occurrence.
However the crash data does show a fair number of more crashes occurring in
Area 4 than conflicts observed in the same area.
􀂃􈌠 The percentage of serious injury crashes (non-incapacitating or worse) for the
signal control group (5.3%) is similar to the percentage for the yield-control group
(4.0%). However, crashes involving incapacitating injuries and fatalities were
found to have occurred at yield-controlled sites only.
􀂃􈌠 Rear-end crashes are dominant at sites with either control type. There was greater
variety of the remaining crash types found to occur at the yield-controlled sites.
􀂃􈌠 There were a couple of anomalies that were noticed upon reviewing the crash data
and rates:
􀂃􈌠 The rates on southbound off-ramp right turn movements from SR 51 at
Greenway and Cactus Roads were very low when compared to other similar
sites (0.10 and 0.40 crashes per million entering vehicles, respectively). It
was determined that although these interchanges had been fully operational
for three years, the connectivity of SR 51 to the north was limited during this
span of time (i.e., the freeway terminated at Bell Road, one mile north of
Greenway Road). The crash rates for these sites were calculated on a
projection of previous off-ramp right turn traffic using current volume data.
However, the current off-ramp right turn volumes are substantially different
now as compared to the three-year assessment period since SR 51 extends past
Bell Road and connects with Loop 101 today. For the three-year assessment
period there was probably very little demand to exit off of southbound SR 51
since the motorist would have just entered onto the freeway one or two miles
north of these sites. A sensitivity analysis of the projected off-ramp right turn
volume used in the crash rate calculations revealed that drastic reductions in
off-ramp right turn volumes to represent the past year conditions only cause
minimal increases in the three-year crash rate average for the sites (no change
in average at the Greenway interchange and a 0.05 increase at the Cactus
interchange).
􀂃􈌠 The next anomaly concerns the extreme difference between the average crash
rate for the signalized northbound off-ramp right turn at Glendale Avenue and
the other signal-controlled off-ramp right turn sites. The average crash rate
for the northbound off-ramp right turn site at Glendale Avenue is about 77%
lower than the average of the other four signal-controlled off-ramp right turn
sites. It does not appear that the lower rate can be attributable to under-reporting
of crashes by the responsible law enforcement because the
southbound off-ramp right turn crash rate at Glendale Avenue is about
52
average for the yield-controlled sites. A more likely theory is based on this
off-ramp right turn movement being the most congested of all the study sites
in the AM peak period. The congestion is caused in part by the cross road
only having two eastbound through lanes—the only study site to have such
configuration (the others have three through lanes on the cross road). The off-ramp
right turn congestion would cause the overall speeds along the off-ramp
right turn lane to be reduced due to extensive queuing, which in turn promotes
a longer time to react to potential conflicts, namely abrupt braking since all
crashes at this site were rear-ends. The cross road congestion also virtually
eliminates the opportunities for off-ramp right turn vehicles to turn right on
red, so that the off-ramp right turn traffic is usually only turning during times
of least potential conflict.
53
CHAPTER 3
OPERATIONAL ASSESSMENT OF DIFFERENT
TYPES OF OFF-RAMP RIGHT TURN CONTROL AT
SINGLE POINT URBAN INTERCHANGES (SPUI) WITHOUT
FRONTAGE ROADS
INTRODUCTION
This chapter describes the approach, process steps, and analysis results from the
operational assessment of different off-ramp right turn controls at SPUIs. The results of
this assessment will be used in concert with the safety evaluation and conclusions
previously presented to develop suggestions on appropriate control types for off-ramp right
turn movement. This information will be presented as a final summarization chapter as
part of this report. Four control type scenarios were examined during the process—two
variations on signal control and two involving yield control. An iterative analysis process
involving a range of off-ramp and interchange volume conditions was used to determine
overall operational effectiveness of each control scenario. Data collected at several SPUI
sites provided actual data that was used to calibrate a micro-simulation model (CORSIM)
that was then used to evaluate numerous combinations of traffic volume conditions and off-ramp
control types that would have not been possible to collect at actual SPUI locations.
CALIBRATION OF CORSIM MODEL
In order to effectively use CORSIM to simulate actual traffic conditions, it is best to use
actual data to calibrate the software parameters governing the model so that it returns
results in line with actual traffic conditions. The data collection undertaken to provide this
data was described in the previous chapter. The base CORSIM model representing a SPUI
(without frontage roads) was calibrated to create six new models representing the six study
sites. The latest version of CORSIM, version 5.1 [13], was used to simulate the
interchange operations because it can produce measures of effectiveness (MOEs), like
control delay, for each movement on a particular link of the network representing the SPUI.
This was particularly important for this project since the evaluation of signalized off-ramp
right turn operations would involve a network link accommodating both the off-ramp left
and right turn movements. Previous versions would not produce output results for control
delay by movement. To promote subsequent comparisons that will be particularly focused
on the effects of the off-ramp right turn control type, all six interchanges were represented
by the same arrangement of network links, except for any network components intended to
vary in order to represent the particular off-ramp right turn control types.
54
Modeling of Off-Ramp Right Turn Lanes
Signal Control
The off-ramp right turn lanes for a particular SPUI model were designed differently
depending on the traffic control used for the movement. For signalized off-ramp right turn
control, the right turn movement shares a network link with the off-ramp left turn
movements. These movements could not be separated onto distinct entry links because
CORSIM is limited to five entry links for the single signal controlling the interchange
model—separate links for the off-ramp right turn movements would have created six entry
links.
Yield Control
In the case of yield controlled off-ramp right turn movements, there was another
adjustment to the model network that was necessary. CORSIM does not allow yield (or
other sign control) and signal controlled movements to operate at the same node (in these
models, the interchange signal control is located at the central node for the network where
all entry and exit links connect). Therefore, to represent yield control of the off-ramp right
turn movement, separate links and nodes were created to represent the off-ramp right turn
lane(s). This accommodation then brought about another item to address. A separate link
serving the off-ramp right turn movement would not be able to be positioned at its realistic
location with respect to the interchange because the relatively close proximity to the center
of the interchange would create a short upstream link as part of the cross street. Gap-acceptance
movements (e.g., yield and right turn on red) in the simulation are driven by
CORSIM’s interpretation of acceptable gaps in the traffic immediately upstream from the
intersection node—in this case, the off-ramp right turn movement intersection node with
the cross street. Very short upstream links are interpreted by CORSIM as a very large gap
when no traffic is present on that link. If this is the case, then the off-ramp right turn
movement would have an unrealistically high movement rate. The only recourse to solve
this issue is to orient the off-ramp right turn movement link so that it intersects with the
cross street a sufficient distance away from the central interchange node. A separation
distance of 610 feet was used and was determined by calculating the equivalent distance for
normal gap acceptance behavior within the CORSIM environment when considering the
cross street traffic traveling at 45 miles per hour.
Dual Off-Ramp Right Turn Movements
When a dual turn movement is provided in CORSIM, the program attempts to balance
traffic volumes between the two lanes making up the turn movement. However, in actual
field conditions, drivers may tend to prefer one lane over another based on future
downstream turn movements, convenience, or preference for turning right from the right-most
lane, especially with right turns on red. Field observations showed that about twice as
many drivers tended to use the right-hand lane when turning right from the off-ramp that
had two right turn lanes regardless of off-ramp right turn control. To account for this
behavior, the two off-ramp right turn lanes were assigned as a “right” lane (for the right-hand
lane) and a “through” lane (for the left-hand lane) although both only allowed for a
55
right turn movement onto the cross street. This convention allowed the proportioning of
the right turn traffic volume between the two lanes according to the field data. The
drawback to this approach is that the left-hand right turn lane would not be permitted to
turn right on red in the signal control scenarios (the yield control scenarios were not
adversely affected), which is not too far removed from actual driver behavior when faced
with a right on red from the left-hand lane of a two lane approach.
CORSIM Parameters and Distributions
For the most part, the default traffic flow parameters used in CORSIM were determined to
provide a reasonable representation of traffic flow at the six modeled interchanges. A few
changes were implemented to further refine the model operations in their simulation of
actual conditions and results. These changes are outlined below.
Turn Speeds
Left turn movement speeds at SPUIs typically are fairly high as compared to a normal
intersection, so the maximum available speed of 44 feet per second was used as the turning
speed for left turn traffic. A right turn speed of 19 feet per second was input for right turns
that shared a network link with left turns. The CORSIM-determined right turn speeds for
links that only accommodated right turn traffic were not modified.
Speed Distribution
A symmetric speed distribution was used in the simulation of the study interchanges in
place of the default distribution typically used in the CORSIM software. This alternative
distribution was used based on a previous study of single point urban interchanges [14].
The mean speed entered for a particular link of the network comprising the simulated
interchange was the 85th percentile speed (posted speed observed in the field review)
divided by the previously observed standard deviation:
(6) 85
1.13
= mean
v v
Using this input information, CORSIM then proceeded to assign speeds to the vehicles in
the simulation using the correct 85th percentile speed.
Traffic Arrival Type
The arrival type for all vehicles was assumed to be random, which was reasonable for the
off-ramp traffic flows. However, the cross street at each interchange is coordinated and
thus would tend to have a more predictable arrival pattern. Without specific data available
or collected for the upstream traffic signals, it was not possible to make any assumptions
about cross street traffic arrival type. Even if an arrival type could be determined,
CORSIM would only allow one arrival type for all vehicles in the simulation, which would
unrealistically affect the off-ramp traffic flows.
56
Field Data Inputs
The field data pertaining to traffic conditions and interchange signal timing obtained from
the data collection effort was input into the models to determine how well they simulated
actual traffic conditions. The simulation with these inputs was observed and examined in
order to determine further adjustments to the model/software to yield realistic results.
Traffic Conditions
Traffic volumes, truck percentages, and turn percentages composed the available data to
enter into the interchange simulation. Turn percentages in CORSIM are limited to the
nearest percentage, so simulated turn volumes do not exactly match the field observations.
Specifically entering the actual turn volumes obtained from the field was possible, but
rejected because even with the actual volumes entered the simulated results would not match
the field results exactly and because the future application of the models for later stages of
analysis would have been made more cumbersome using this method of volume input.
Signal Timings and Coordination
The six study interchanges are all currently part of coordinated signal systems. As a result,
each of the six sites has a fixed signal cycle length. Since a fully actuated intersection does
not have a fixed cycle length, unless the cycle length is constrained, they were modeled using
the time-based coordination feature in CORSIM. The coordination for each simulated
interchange was programmed using the phase times and splits from the interchange timing
sheets obtained during the overall data collection task for the project. The “offset” value
associated with time-based coordination was not applicable and thus set to zero for simplicity
since no other data was available for other upstream coordinated traffic signals.
A few adjustments were necessary to allow CORSIM to accept the actual timings and splits
used in the field. For example, many movements had no minimum green times shown on the
timing sheets, but zero or some very small value (such as one second) could either not be
entered or would produce unrealistically short phase durations, respectively. Therefore, for
the phases that did not have a specified minimum green time, 8 seconds was used instead and
appeared to provide the best compromise between the controller settings and the observed
phase durations in the field.
Phases 2 and 6 represented westbound and eastbound through movements, respectively, at
five of the six interchanges. However, at the Loop 202/Rural Road interchange the off-ramp
movements are oriented westbound and eastbound and thus do not have a through movement
nor Phases 2 and 6, which caused a mismatching of phase numbering and inter-change
movement type. In order to keep the same phase assignments for all of the inter-changes, the
phase structure of the Loop 202/Rural Road interchange was “reassigned” so that Phases 2
57
and 6 were the through movements on Rural Road rather than Phases 4 and 8. All of the
other phases at this interchange were altered accordingly, as shown in Figure 17.
Original Phases Reassigned Phases
Off-Ramps Rural Road Off-Ramps Rural Road
1 3 4 3 1 2
5 7 8 7 5 6
Figure 17. Reassigned Phases for Loop 202/Rural Road Interchange
The final step in the CORSIM model calibration process was to observe the simulation
operations. If the simulated phases did not reasonably match the field observations of
green phase duration, then the coordination parameters were further adjusted until either
reasonable agreement was obtained or the limits of the other controller settings (e.g.,
maximum green time) were reached.
58
CALIBRATION RESULTS
The calibration process focused on three interchange parameters which were used to
adjust the modeled interchange operations and ultimately served as the basis for
comparing the model results with the actual field data/results. The three parameters are
the off-ramp right turn delay, green phase durations, and percent of off-ramp right turn
vehicles stopping. All six interchanges were analyzed for both the morning and evening
peak hours.
Delay
Figure 18 shows the relationship between the off-ramp right turn control delay obtained
from the model simulation and the field observations. In general, the predicted off-ramp
right turn delay from the model was somewhat less than the observed delay. In part, this
is due to CORSIM allowing turns to be completed in shorter gaps than most drivers
would typically use and because of the 610-foot spacing between the interchange signal
and the yield controlled off-ramp right turn movements. Adjustments to the gap
acceptance distribution and the follow-up time for the off-ramp right turn traffic had little
effect on the overall tendency of the model to underestimate off-ramp right turn delay. In
addition, increasing the follow-up time would randomly cause oversaturation in the off-ramp
right turn lane(s) which would result in large variations in simulated delay after
only a small change in the follow-up time value. Therefore, the default model values
were retained.
Figure 18. Comparison of Simulated and Field-Measured Delays for the Off-Ramp
Right Turn Movement
0
5
10
15
20
25
30
35
40
45
50
0 5 10 15 20 25 30 35 40 45 50
Measured Right Turn Delay (s/veh)
Simulated Right Turn Delay (s/veh) AM Peak Hour PM Peak Hour
59
Green Phase Duration
The green phase durations for the cross street through and left turn movements as well as
the off-ramp left turn (and right turn when signalized) movements observed in the field
compared very closely with the simulated values as shown in Figure 19. This agreement
is a result of being able to directly manipulate these values as part of the data input
process for the model. The green phase durations longer than 30 seconds shown in
Figure 19 were all for the cross street through movements. Since these movements were
associated with coordinated signal phases, they would acquire any extra green time that
was not used by other movements during a particular signal cycle length. Thus, the
variation in the green phase duration of the cross street through phases is higher than the
other phases.
Figure 19. Comparison of Simulated and Field-Measured Green Phase Durations
for the Off-Ramp Right Turn Movement
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
Measured Green Phase Duration (s)
Simulated Green Phase Duration (s)
AM Peak Hour PM Peak Hour
60
Percentage of Off-Ramp Right Turn Vehicles Stopping
Figure 20 shows the relationship between the percentage of off-ramp right turn vehicles
stopping in the simulations and in field observations. The relationship shown in the
figure is not very strong for several reasons. First, the number of vehicles that have to
stop when turning right depends on the current signal phase, traffic from other
movements, and whether previously arriving off-ramp right turn vehicles have stopped
and are in a queue. The simulation will never be able to match field-observed conditions
in this regard. Second, the definition of a “stop” in CORSIM is very restrictive, requiring
the simulated vehicle to come to a complete stop. A field-observed stop was based on the
definition provided in the Highway Capacity Manual [12] which only considers a stop to
be when a vehicle has come within a vehicle length of a stopped vehicle and intends to
stop itself. Simulated vehicles that roll through a yield sign (or a right turn on red) may
not be considered as fully stopped but might have been considered differently in the field.
Third, CORSIM may be allowing vehicles to make right turns on red at times when
actual drivers would not consider such maneuvers. An example is when CORSIM allows
a vehicle to turn right on red when a suitable gap is found in the traffic stream on the
approaching link even if this approaching traffic is the beginning of a queue discharge.
Figure 20. Comparison of Simulated and Field-Measured Percentage of Vehicles Stopping
for the Off-Ramp Right Turn Movement
0
20
40
60
80
100
0 20 40 60 80 100
Measured Percentage Right Turn Vehicles
Stopping
Simulated Percentage Right Turn
Vehicles Stopping
AM Peak Hour PM Peak Hour
61
Analysis of Off-Ramp Right Turn Control Types
The previous chapter presented some analysis and results pertaining to off-ramp right turn
operations for the actual study sites. Although those results and determinations are based
on actual field data, they are limited in scope to only six SPUI sites. In order to draw
broader conclusions concerning the effects of different types of control on the off-ramp
right turn movement, more samples and data are required. To facilitate this need a massive
amount of field data would have to be collected and processed or the limited real data can
be used to develop a working model of SPUIs where the off-ramp right turn control could
be varied, as is done in this project. The calibration of the model parameters based on
actual field data allows for deviation from the replicated field conditions to
experimental/hypothetical situations. This then allowed for the testing of other forms of
control while having some confidence that the results would be representative of actual
traffic conditions under the same conditions. This section explains the process and results
of conducting these analyses using the calibrated CORSIM model of a SPUI.
Off-Ramp Right Turn Control Types
The first step in the analysis process is to determine what control types will be evaluated
and contrasted. The two prominent off-ramp right turn traffic control types used in the
Phoenix area are signal control and yield control. Therefore, the control types evaluated in
the analysis would focus only on these two control types and disregard other options such
as stop control or free flow/merge. Within the signal and yield control types, there are
other factors that would affect the operation of the off-ramp right turn movement, such as
number of right turn lanes, vehicle detection usage/presence, and signal phasing. There are
four control types (two variations of signal and yield control) examined in the analysis,
which equate to eight control scenarios when considering each control type. Each variation
would have two versions for one and two off-ramp right turn lanes. Each control type is
described below.
Signal Controlled Off-Ramp Right Turn
The off-ramp right turn movement can be controlled by signal indications much as any
other intersection movement is controlled. The signal head(s) will indicate a green right
turn arrow during the portion of the signal cycle when the off-ramp right turn movement is
considered protected—in other words during the adjacent cross street left turn phase. At all
other times, a red indication would be displayed to off-ramp right turn traffic requiring that
the right turn traffic stop and check traffic conditions before turning right (unless otherwise
posted, although postings of this nature were not present at the study sites).
This control type has two variations that were assessed in the model analyses. The
variations concern the allotment of signal phasing to the off-ramp right turn traffic. One
version only gives a green arrow indicat