Table Of Content

TRAFFIC ENGINEERING WHAT? WHY? HOW? ARIZONA HIGHWAY DEPARTMENT TRAFFIC ENGINEERING DIVISION ARIZONA HIGHWAY COMMISSION Forrest C.Braden Chairman Bus Mead Vice-Chairman Ed C.Locklear Member Peter B.WiJharm Member DanC.McKinney Member Justin Herman State Highway Director William N. Price State Highway Engineer David L.Kennison Acting Traffic Civil Engineer Ronald D.Midkiff Traffic Designer February 1968 l-ITHO~"APHEDBY ARIZONA HIG~"AYOEPARTMENT REPRODuCTION DIV"i'ON TABLE OF CONTENTS Pllge What isTrattic Engineering? . 1 Why is Traffic Engineering Essential? ... ,..... 2 HowDoesthe Traffic Engineer Solve Traffic Problems? 3 How Doesthe Traffic Engineer Promote SaferTraffic Operation? , . . . . . 4 What are TrafficControl Devices? . 5 Whatis Meant byUniformityof TrafficControl Devices? 6 Howare Speed Limits Determined? .... .. 8 What EffectDoPosted Speed Limits Haveon Actual Traffic Speeds? " 9 When ShouldTraffic Signals BeInstalled? 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 DoArizona Laws Say About Angle Parking? 18 WhyParallel Curb Parking? 19 DoesProhibition of All Curb Parking HelpTraffic Movement? .... . . . . . . . . . . .. . . . . . . 20 WHAT IS TRAFFIC ENGINEERING? Traffic Engineering is the application of engi neering 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 engineer ingapplies thephysical laws ofmotion tothemotor vehicle, the roadway, and the driver, while bring ing into playa knowledge of psychology and the habits of the motoring public. -1- WHY IS TRAFFIC HOW DOES THE TRAFFIC ENGINEER ENGINEERING ESSENTIAL? SOLVE TRAFFIC PROBLEMS? Many persons still wonder why a traffic prob The role of the traffic engineer may be com lem is sodifficult that anengineer should be called pared to that of the medical profession in protect upon for a solution. Why not just install a traffic ing the public. As a trained professional he looks signal, or lower the speed limit, or erect more atthesymptoms. and inorderto makeacompetent signs? diagnosis he makes traffic counts, analyzes acci One of the greatest obstacles a professional dentstatistics, studies speed data, examines road traffic engineer faces inapplying sound principles way conditions, conducts research, and studies of traffic engineering isthe fact that "everyone is what other professionais are doing and the results a traffic expert!" The unfortunate result of this they have achieved, attitude of expertise is the creation of traffic haz ardswhen false theories of individuals or groups Just as the doctor's decision is accepted in are put intoeffect. matters regarding health. even though the medi Whenever unnecessary or excessive traffic cine may be bitter or the needle painful, so should controls areinstalled, hazardous traffic conditions the decision of the professional traffic engineer usually result. begiventhe prime consideration. -2- -3- HOW DOES THE TRAFFIC ENGINEER WHAT ARE TRAFFIC PROMOTE SAFER TRAFFIC CONDITION? i i CONTROL DEVICES? ·8 Experience has shown that disrupt ing the smooth flow .-:~. of traffic will in crease the probabil ity of accidents. Er ratic traffic operation may be created by vehicles stopping or slowing in theroadway.pass ing and weaving maneuvers, or driver surprise elements. For ex· ample. unwarranted traffic signals, unreasonably low speed limits. and too many signs cause driver confusion and indecision. Traffic Control devices are all signs, signals, Slower speed doesn't always mean safer traf fic operation. The chances of a driver becoming markings, and devices placed on, or adjacent to, involved in an accident are least when he is a street or highway by a public body having traveling at the average speed of traffic. authority to regulate, warn, or guide traffic. -4- -5- 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. NO Uniformity aids police, courts and road users PASSING 15 by giving everyone the same interpretation, It SC 1MSESSION aids public highway officials through economy in manufacture, installation, maintenance, and prepared bya National Committee which included administration. state, county, and municipal representation. The " Manual on Uniform Traffic Control The standards in this Manual with certain Devices" is the publication that sets forth the exceptions have been adopted for use in Arizona basic princir'~s which govern the design and us andapplytoall streets and highwaysregardlessof age of traffic control devices. The Manual was the governmental agency having jurisdiction. - 6- - 7- HOW ARE SPEED LIMITS WHAT EFFECT DO POSTED SPEED DETERMINED? LIMITS HAVE ON ACTUAL TRAFFIC SPEEDS? Legal speed limits are established by Arizona law and may be changed only when justified on Very little effect. There is a common belief the basis of an engineering study. among laymen, and even by some officials, that A widely accepted principle is to set speed the mere posting of speed limit signs will cause drivers to react accordingly. This is not true and limits as near as practicable to the speed below is why posted speed limits must be realistic to which 85% of the vehicles are traveling on the receive compliance. highway. Experience has shown that approxi Unrealistically low speed limits will invite mately 85% of the motorists drive at a speed that violation by responsible drivers. Enforcement of is reasonable and prudent. unreasonably low limits sets up the so-called Speed limits thus established encourage vol "speed trap," which results in poor public untary compliance because they appear reason relations. able to the public. Those 15% of drivers who will The posting of proper speed limits has the not comply with reasonable speed limits are the beneficial effects of smoothing traffic flow and drivers who are subject to enforcement action. aiding effective law enforcement. -8- -9- WHEN SHOULD TRAFFIC SIGNALS BE INSTALLED? Traffic signals should be installed when they will alleviate more problems than they will create. An unwarranted traffic signal can result in in creaseddelay,congestion, and accidents. Another common belief is that traffic signals are the answer to all traffic problems at inter sections. If this were true, no traffic engineer in his right mind would 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 road an accident potential is created. It does not matter whether the stop is caused by a flat tire, a left turn into a driveway, ing hopelessness of waiting in a long line of cars or by a traffic signal - the possibility exists that for a traffic signal to change, moving ahead a a following motorist will not notice the stopped few feel. and then having the signal turn red vehicle until it is too late. again? What motorist has not experienced that sick Our whole economy is geared to the basic ening feeling that occurs when a traffic signal objective of keeping vehicles moving and every suddenly turns amber a few hundred feet in front traffic signal installed on a highway detracts from of him? Who has not experienced the aggravat- this movement. - 10 - -11 -