TRAFFIC ENGINEERING
WHAT? WHY? HOW?
ARIZONA HIGHWAY DEPARTMENT
TRAFFIC ENGINEERING DIVISION
ARIZONA HIGHWAY COMMISSION
Forrest C. Braden
Bus Mead
Ed C. Locklear
Peter B. WiJharm
Dan C. McKinney
Justin Herman
William N. Price
Chairman
Vice-Chairman
Member
Member
Member
State Highway Director
State Highway Engineer
David L. Kennison
Acting Traffic Civil Engineer
Ronald D. Midkiff
Traffic Designer
February 1968
l-ITHO~"APHED BY ARIZONA HIG~"AY OEPARTMENT
REPRODuCTION DIV"i'ON
TABLE OF CONTENTS
What is Trattic Engineering? .
Why is Traffic Engineering Essential? ... , .....
Pllge
1
2
How Does the Traffic Engineer Solve Traffic Problems? 3
How Does the Traffic Engineer Promote Safer Traffic
Operation? , . . . . . 4
What are Traffic Control Devices? . 5
What is Meant by Uniformity of Traffic Control Devices? 6
How are Speed Limits Determined? .... .. 8
What Effect Do Posted Speed Limits Have on Actual
Traffic Speeds? " 9
When Should Traffic Signals Be Installed? 10
What is the Primary Purpose of Guide Signs? . . 12
How are Guide Sign Messages Determined? 14
Is Angle Curb Parking a Vanishing Custom? 16
What Do Arizona Laws Say About Angle Parking? 18
Why Parallel Curb Parking? 19
Does Prohibition of All Curb Parking Help Traffic
Movement? .... . . . . . . . . . . . . . . . . . . 20
WHAT IS TRAFFIC ENGINEERING?
Traffic Engineering is the application of engineering
principles to achieve safe, efficient, and
convenient traffic operation. Traffic Engineering
deals with the movement of motor vehicles and
the methods used to direct them to their
destination.
From a scientific standpoint, traffic engineeringapplies
the physical laws of motion to the motor
vehicle, the roadway, and the driver, while bringing
into playa knowledge of psychology and the
habits of the motoring public.
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WHY IS TRAFFIC
ENGINEERING ESSENTIAL?
Many persons still wonder why a traffic problem
is so difficult that an engineer should be called
upon for a solution. Why not just install a traffic
signal, or lower the speed limit, or erect more
signs?
One of the greatest obstacles a professional
traffic engineer faces in applying sound principles
of traffic engineering is the fact that "everyone is
a traffic expert!" The unfortunate result of this
attitude of expertise is the creation of traffic hazards
when false theories of individuals or groups
are put into effect.
Whenever unnecessary or excessive traffic
controls are installed, hazardous traffic conditions
usually result.
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HOW DOES THE TRAFFIC ENGINEER
SOLVE TRAFFIC PROBLEMS?
The role of the traffic engineer may be compared
to that of the medical profession in protecting
the public. As a trained professional he looks
atthe symptoms. and in order to make a competent
diagnosis he makes traffic counts, analyzes accident
statistics, studies speed data, examines roadway
conditions, conducts research, and studies
what other professionais are doing and the results
they have achieved,
Just as the doctor's decision is accepted in
matters regarding health. even though the medicine
may be bitter or the needle painful, so should
the decision of the professional traffic engineer
be given the prime consideration.
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Exper ience has
shown that disrupting
the smooth flow
of t r aff ic wil l increase
the probability
of accidents.
HOW DOES THE TRAFFIC ENGINEER
PROMOTE SAFER TRAiFFIC.i-CO:·~N.8DITION?
Er r a t ic tra ffic
operation may be
created by vehicles
stop ping or slowing
in t he roadway. passing
a n d weav ing
maneuvers, or driver surprise elements. For ex·
ample. unwarranted traffic signals, unreasonably
low speed limit s. and too many signs cause driver
confusion and indecision.
Slower speed doesn' t always mean safer t raf fic
operation. The chances of a driver becoming
involv ed in an acc ident are least whe n he is
tra veling at the average speed of t raffic.
-4-
WHAT ARE TRAFFIC
CONTROL DEVICES?
Traff ic Control devices are all signs, signals,
markings, and devices placed on, or adjacent to,
a st ree t or highway by a pub lic body having
authority to regulate , warn, or guide t raffic.
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WHAT IS MEANT BY UNIFORMITY OF
TRAFFIC CONTROL DEVICES?
Uniformity means treating similar situations
in the same way, This simplifies the task of the
driver because it aids in instant recognition and
understanding.
Uniformity aids police, courts and road users
by giving everyone the same interpretation , It
aids public highway off icials th rough economy
in manufacture, installation , maintenance, and
administration.
The " Manual on Uniform Traffi c Cont ro l
Devices" is the publication that sets forth the
basic prin cir ' ~ s which govern the design and usage
of traff ic control devices. The Manual was
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NO
PASSING
15
SC
1MSESSION
prepared by a National Committee which included
state, county, and municipal representation.
The standards in this Manual with certain
exceptions have been adopted for use in Arizona
and apply to all streets and highways regardless of
the governmental agency having jurisdiction .
- 7 -
HOW ARE SPEED LIMITS
DETERMINED?
Legal speed limits are established by Arizona
law and may be changed only when justified on
the basis of an engineering study.
A widely accepted principle is to set speed
limits as near as practicable to the speed below
which 85% of the vehicles are traveling on the
highway. Experience has shown that approximately
85% of the motorists drive at a speed that
is reasonable and prudent.
Speed limits thus established encourage voluntary
compliance because they appear reasonable
to the public. Those 15% of drivers who will
not comply with reasonable speed limits are the
drivers who are subject to enforcement action.
-8-
WHAT EFFECT DO POSTED SPEED
LIMITS HAVE ON ACTUAL
TRAFFIC SPEEDS?
Very little effect. There is a common belief
among laymen, and even by some officials, that
the mere posting of speed limit signs will cause
drivers to react accordingly. This is not true and
is why posted speed limits must be realistic to
receive compliance.
Unrealistically low speed limits will invite
violation by responsible drivers. Enforcement of
unreasonably low limits sets up the so-called
"speed trap," which results in poor public
relations.
The posting of proper speed limits has the
beneficial effects of smoothing traffic flow and
aiding effective law enforcement.
-9-
WHEN SHOULD TRAFFIC SIGNALS
BE INSTALLED?
Traffic signals should be installed when they
will all eviate more problems than they will create .
An unwarranted traffic signal can result in increased
delay, congestion, and accidents .
Another common belief is that tra ffic signals
are the answer to all traffic problems at intersections
. If this were t rue, no traffic engineer
in his r ig ht mind wou ld deny a request tor a
signal.
However, a traffic signal only functions by
stopping traffic. and any time a motor vehicle is
stopped in the roa d an accident po tential is
created. It does not matter whether the stop is
caused by a fla t tire, a left turn into a driveway,
or by a t raff ic signal - the possibility exists that
a followi ng motorist will not notice the stopped
vehicle until it is too late.
What motorist has not experienced that sickening
feeling that occurs when a traff ic signal
suddenly turns amber a few hundred feet in front
of him? Who has not experienced the aggravat -
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ing hopelessness of waiting in a long line of car s
for a traffic signal to change, moving ahead a
few feel. and then having the signal turn red
again?
Our whole economy is geared to the basic
objective of keeping vehicles moving and every
traffic signal installed on a highway det racts from
this movement.
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ROUTE MARKERS
DIRECTION SIGNS
,
HOSPITAL •
DOUGLAS
CITY LIMITS
40
t PHOENIX
+-TUCSON
I~ TER ST [
PRINCE RD
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INFORMATIONAL SIGNS
HIS TOR IC AL
MARKER
Y2 MILE
Northern Ave
EXIT ' MI LE
DEPOSIT
LITTER
1/4 MILE
JEROME 22
.CLARKDALE 27
,
-12-
WHAT IS THE PRIMARY PURPOSE OF
GUIDE SIGNS?
The principle purpose
of guide signs
is to direct motorists
to their destinations
by the best rou te .
However, it is not
fe asible to ins ta ll
signs listing all of
the possible destina- ....
ti on s that may be
reached f rom th e ~
highway.
Drivers must be expected to make reasonable
preparation for locating their destination
and to have information tha t is readily available
on road maps.
HOW ARE GUIDE SIGN MESSAGES
DETERMINED?
Simplicity and clarity are necessary because
drivers of moving vehicles are unable to read
lengthy or complicated messages on signs. For
th is reason, the number of lines on a sign is kept
to a minimum , and in no case exceeds three.
On f reeways, high traffic speeds demand that
the number of signs be lim ited to those absolutely
essential for the guidance of the motorist.
Freeway ex its are id ent if ied by t he ro ute
number or by the name of the intersecti ng road.
Cert ain additiona l messages may be provided
where just ifie d, although in urban areas no other
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EXIT " ;,.;
25 t~
M.P. H. . "1
»: ""'!S
e. ~
places are normally shown except airports having
regularly scheduled commercial air travel.
In rural areas signs may be installed to direct
motorists to services such as roadside rests, gas,
food, and lodging.
-15-
IS ANGLE CURB PARKING A
VANISHING CUSTOM? :111 ! i l
Q ~
I :i~@ ~1i~i:\ t~~:1!!~ i ii~l . ~. j
9 ~. I:J [J
~ i ! -• .~ ~I · ~~ g
0{ t2 ~~
Yes. As the automobile
has replaced
the horse and buggy,
parallel curb park ing
has replaced ang le
curb park ing. In the
early days of th e
horse-drawn vehicle
and the Model T, the
custom was to par k
at the " h i t c hi ng
post" at an angle to
the cu rb . In many cornrnunmes trus t radit ional
custom remains , though horses have disappeared
from the local tra ffic scene.
No one questions the fact that ang le curb parking
accommodate s more vehicles than para llel curb
parking, but it does so at the expense of street
space and the movement of traffic.
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An tle of Widttl Needed
Parking Width of Street lot Parking
at Curb UsedWilen Parked Plus Maneu~e r ln il
Parallel 8.0 ft. 20.0 It.
45 degrees 19.1 It. 33.1 ft .
60 degrees 20.4 It. 39.4 It.
90 degrees 19.0 ft . 45 .0 ft .
For example, if 45" angle park ing exists on
both sides. a street as wide as 70 feet will be congested
with not even a single lane of traffic moving
fr eely in either direction when cars are maneuvering
into park ing stalls on both sides at once. Few
cities have st reets of 70 feet width or more; therefor
e, co nt inuance of ang le par king on through
arteries is rarely just ified.
Angle park ing is considerably more hazardous
to pedestrians and motorists because it involves
backing into approa ching t rafiie.
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WHAT DO ARIZONA LAWS SAY
ABOUT ANGLE PARKING?
Section 28-874 of the Arizona Revised Statutes
states:
(a) "Except as otherwise provided in this section
every vehicle stopped or parked upon a roadway
where there are adjacent curbs shall be so
stopped or parked with the right-hand wheels of
the vehicle parallel to and within eighteen inches of
the right-hand curb."
(b) "Local authorities may by ordinance permit
angle parking on any roadway, except that angle
parking shall not be permitted on any federal-aid
or State highway unless the commission has determined
by resolution or order entered in its minutes
that the roadway is of sufficient width to permit
angle parking without interlering with the free
movement of traffic."
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WHY PARALLEL CURB PARKING?
A legal responsibility.
Fewer fatalities, injuries, accidents regardless
of street width.
More street space for moving vehicles.
Modern and orderly appearance of business
area.
less encroachment on sidewalks.
Less interference with emergency vehicles.
More convenient and safer for vehicle passengers.
Better sight distance at intersections and
crosswalks.
Better visibility of street and highway signs.
-19-
DOES PROHIBITION OF ALL CURB
PARKING HELP TRAFFIC
MOVEMENT?
Prohibition of all crub parking, where rigidly
enforced, provides greatly increased capacity and
safety.
The removal of curb parking and the installation
of a painted median island with left turn bays
on a section of U. S. 80 in Yuma reduced accidents
by 40%.
Curb parkingprohibition can do more than any
other traffic regulation to increase the capacity of
a street for traffic movement.
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