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Traffic - Frequently Asked Questions

Traffic Engineering


T.E. Book CoverQ:  What is traffic engineering?


A:  Traffic Engineering is the field of engineering which deals with the planning, design, construction, and operation of roads, streets, and highways, their networks, terminals, access to abutting lands and relationships with other modes of transportation for the achievement of safe, efficient, and convenient movement of persons and goods.  Traffic Engineering applies engineering principles to existing transportation facilities to help solve transportation problems, and takes in account the knowledge of psychology and habits of users of the transportation systems.


MUTCD CoverQ:  What is the Manual on Uniform Traffic Control Devices?


A:  The Manual on Uniform Traffic Control Devices, or MUTCD, is a set of standards issued by the Federal Highway Administration.  The MUTCD was developed to ensure that all traffic signs are identical and comply with the same standards.


Traffic Signals


Signal HeadQ:  When should a traffic signal be installed?


A:  Traffic signals should only be installed when they are warranted as determined by an engineering study.  A warranted traffic signal that is properly located and operated may provide for more orderly movement of traffic, and may reduce the occurrence of certain types of crashes. On the other hand, an unwarranted traffic signal can result in increased delay, congestion, and crashes. Installations are based on engineering studies that consider the following:



  • Excessive side street volumes

  • Pedestrian delays and/or pedestrian safety

  • Crash history

  • Flow of traffic along major street

  • Side street delays

  • Uniform traffic flow

Q:  What is traffic signal coordination?


A:  Traffic signal coordination is when two or more traffic signals are working together so that cars moving through the group of signals will make the least number of stops possible.


Red SignalQ:  Does traffic signal coordination mean that I will never have to stop for a red light?


A:  No.  There are many reasons why you will still have to stop at red lights.  Each of the reasons has to do with the amount of time available for the green light in your direction.



  • Pedestrian crossings—For safety, enough time must be allowed for a pedestrian to cross the street from curb-to-curb, walking at a pace of about three to four feet per second.

  • Cross traffic—Like pedestrian crossings, enough time must be allocated to clear the waiting traffic on the cross street.  The heavier the cross traffic, such as is experienced near schools and businesses, the more time that is needed to clear them through the intersection and the less time that is available for the green light in the “coordinated” direction.

  • Left-turn signals—Where left-turning traffic is especially heavy and/or the amount of opposing traffic is so heavy that there are not enough gaps in the traffic to safely complete a left-turn, a protected left-turn signal is usually installed.  The amount of time for protected left-turning traffic also limits the time permitted for the “through” traffic flow in the opposite direction.

  • Two-way traffic flow—The distance between traffic signals and the speed of the traffic determine the way in which the green lights at the next traffic signal align.  When the spacing is not equal between traffic signals, the green lights may only line up well in one direction.  When this happens, the City tries to line up the green lights in the direction with the most traffic.  The traffic in the other direction may have to stop occasionally as a result.

  • Off-peak traffic periods—Traffic signals are not coordinated 24 hours a day.  During times when traffic is light, traffic signals are usually allowed to run independently.  Traffic signals are most often coordinated during the “peak” travel times when traffic is heaviest, usually between 7:00 AM and 9:00 PM.

Q:  Why do I have to wait so long for a green light on a side street?


A:  In order to have coordinated traffic signals, each traffic signal in the group must be able to allow the green light for all movements during a common fixed time period.  The time period chosen is usually determined by the largest intersection with the most different movements.  This will most often be an intersection that has protected left-turn arrows for all directions and wide cross streets.  For that reason, the time period that is fixed for each traffic signal (the cycle length) may be rather long.  So, if you are waiting for a green light to cross the “coordinated” street where there are protected left-turn arrows and there is very light traffic on the side street, chances are good that you will feel like you are waiting for a long time, even though you should rarely have to wait any longer than about two minutes.


Signs


STOP SignQ:  Will more STOP signs slow traffic on a street?


A:  Many requests are received for STOP signs to interrupt traffic or to slow speeding vehicles.  However, studies across the nation show that there have been a high number of intentional violations when STOP signs are installed as nuisances or “speedbreakers.”  While studies show that speed is reduced in the immediate vicinity of unwarranted STOP signs, speeds are often higher between intersections than before the unwarranted signs were installed.  This is caused by motorists “making up for lost time.”


Q:  Who decides where to locate signs along our roadways?  Couldn’t we have more safety and directional guide signs along our roads?


A:  Guidelines for the size, shape, color, wording, symbols and location of traffic signs on state roads are set forth in the Manual of Uniform Traffic Control Devices (MUTCD).  This manual sets forth the basic principles that govern the design and use of traffic control devices.  According to the manual, to be effective a device should fulfill a need, command attention, convey a clear, simple meaning, command respect of road users, and give adequate time for proper response.  Any additional safety and directional signs would have to meet these standards and fall within the specific guidelines for each device.  Too many signs would fail to convey the clear, simple message that is intended for the motorists.


Pavement Markings


Pavement MarkingsQ:  When is striping provided in the center of the roadway?


A:  Centerline markings shall be placed on paved streets and highways as follows:



  • All rural arterials and collectors with a travel way 18 feet or more in width and Average Daily Traffic Volumes of 1,000 vehicles per day or greater.

  • All urban arterials and collectors with a travel way 20 feet or more in width and Average Daily Traffic Volumes of 5,000 vehicles per day or greater.

  • All two-way streets and highways having three or more travel lanes.

Centerline markings should be placed on paved streets and highways as follows:



  • Urban arterials and collectors with a travel way of 20 feet and Average Daily Traffic Volumes of 2,500 vehicles per day or greater.

  • At other locations where an engineering study indicates a need for them.

Centerline markings may be placed on paved streets and highways with a width of 16 feet or greater.


Q:  When is striping provided on the edges of the roadway?


A:  Edgeline markings shall be placed on all rural arterials with a travel way 20 feet or more in width and an Average Daily Traffic Volume of 1,000 vehicles per day or higher.


Edgeline markings should be placed on streets as follows:



  • Rural collectors with a travel way of 20 feet or more in width and where the edge of the travel way is not otherwise marked with curbs or other pavement markings.

  • At other locations where an engineering study indicates a need for them.

Edgeline markings may be placed on streets with or without marked center lines.


Traffic Calming


RoundaboutQ:  What is traffic calming?


A:  Traffic calming slows speeding traffic on residential streets without restricting access to them.


Q:  Is any road a candidate for traffic calming?


A:  Traffic calming applies only to existing streets.  It does not apply to future roads or subdivision streets under construction.  It also does not apply to high-speed, high-volume roads.


Q:  How long is the process from the time the request for traffic calming is made initially?


A:  The entire process takes approximately one year assuming that no delays are encountered along the way.


Speed Limits


Speed Limit SignQ:  How are speed limits determined?


A:  City ordinance sets the prima facie speed limit in Norman based upon the type of roadway under investigation.  Motorists are to assume this prima facie limit is in effect unless there are signs posting a different speed limit.  Speeds greater that the prima facie speed limit can be set on the roadways by the City Transportation Engineer, or his designee, based upon engineering studies that incorporate the following factors:



  • Prevailing speed of motorists as determined in a speed study

  • Roadway development including land use type, driveway spacing, and parking practice

  • Crash experience

  • Function classification of the streets

  • Roadway characteristics including alignment, surface, grade, sight distance, and design speed

Q:  Will a lower speed limit help reduce speeding?


A:  No.  Research conducted throughout the country over several decades has shown that drivers are influenced by the type of street and current traffic conditions, and not the posted speed limit.