Traffic Signals

Traffic Signals and Signal Coordination (Timing) 

General Information

Traffic Signals are a vital tool used  to safely and efficiently manage vehicle, bicycle and pedestrian traffic on state highways.

To achieve optimum efficiency, traffic signals must be monitored and adjusted to serve changing traffic patterns.

Traffic engineers collect detailed information about:

  • traffic patterns
  • volumes
  • speeds

Once this data is analyzed, new timing plans are developed and field adjustments are implemented as required.

To maximize traffic flow on arterials and along corridors, closely spaced signals are inter-connected, creating coordinated signal systems .

Using traffic signals in coordinated systems may benefit travelers by:

  • reducing time delay
  • providing improved safety
  • efficient use of fossil fuels
  • reduced air pollution

Traffic Signals and Signal Coordination (Timing)

What does it take to get a new traffic signal installed?
Traffic counts and accident statistics are the primary considerations for installing traffic signals. When they are installed, traffic signals provide a solution to specific operational challenges, such as stopping heavy flow of traffic on a major roadway to permit crossing movements from intersecting minor streets.

When programmed for optimum timing efficiency, signals can increase the traffic handling capacity of an intersection, and can reduce the occurrence of angle, or 'broadside' collisions. However, they are not the solution to all traffic woes. Most people don't realize that rear-end accidents can increase when a traffic signal is installed.

Traffic signals cause accidents?
Rear-end collisions usually increase when a signal is installed. Normally, traffic engineers are willing to trade off an increase in rear-end collisions for a decrease in the more severe angle-type accidents. However, when there is no angle-type accident problem at an intersection, a traffic signal may actually raise the number of accidents in a given area.

Is it true that traffic signals always make traffic flow smoother and safer?
No. They only make traffic flow smoother and safer when used in proper situations. Traffic signals cause traffic to stop where it may not have had to stop before. When used at an intersection where not justified, signals can cause frustration in drivers, who then seek alternate routes.

These routes usually are not built to handle increased traffic flow. In addition, drivers frustrated by unnecessarily long waits at signals may begin to disobey the law. Traffic control devices are most effective when perceived as reasonable by the motorists, bicyclists, and pedestrians that use them.

When do traffic engineers decide signals are justified?
Usually after lesser forms of control, such as stop signs or yield signs have proven to be ineffective. Then traffic engineers follow specific, uniform guidelines to determine whether a traffic signal is necessary.

What about intersections that don't meet this engineering criteria?
Problems can occur. Signals almost always create overall delay to drivers. In fact, minor side street traffic may experience excessive delay, particularly during off-peak hours.

Because of this, drivers may actually avoid the signalized intersection and switch to alternate routes or, to residential streets not designed to handle through traffic. People also seldom consider the cost of signals, both in public funds and out of their own pockets.

Out of pocket costs to me?
It costs the taxpayer $250,000 to $500,000 to purchase and install a traffic signal. Electric bills and routine maintenance amount to about $8,000 a year. Drivers also have increased costs for fuel, time delay, and accidents. This adds to the reasons for installing signals only where clearly justified.

If I think a signal may be needed at an intersection, what should I do?  
Contact the appropriate public agency: WSDOT for state highways, or your city or county public works department for local roadways. Ask the traffic engineers to review available data on the intersection, and to consider initiating a more detailed study to see if a serious problem indeed exists.

Talk to them about the possibility of trying lesser forms of traffic control, such as improved signing and pavement markings, or minor intersection improvements to see if that alleviates the problem. Working together on the safest, most appropriate solutions is the best approach to keeping traffic flowing safely and smoothly in our communities.

Traffic signal coordination

Traffic signal coordination occurs when a group of two or more traffic signals are working together so that cars moving through the group will make the least number of stops possible. In order for this to happen, each traffic signal in the group must allow a green light for all directions of travel during a fixed time period.

In addition, that fixed time period must be the same for each traffic signal in the group. Since each traffic signal in a group runs through all its directions in the same time period, it then becomes possible to "line up" the green lights for one direction. The way the green lights "line up" depends on the distance between traffic signals and the speed of the traffic.

Does this mean I will never have to stop for a red light?

Unfortunately, the answer to this question is "No." There are many reasons why, even when traffic signals are coordinated, 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. In order to operate traffic signals safely, several things must be considered.

Because of the fixed amount of time for the "coordinated" traffic signal to provide a green light for all of the traffic movements, each of the following has a direct relationship to the amount of time available for the green light at each traffic signal within a coordinated group along a roadway.

Pedestrian Crossing: For safety, enough time must be allowed for a pedestrian to cross the street from curb-to-curb walking at a pace of four (4) feet per second*. This is called the pedestrian clearance interval and is represented by the flashing "DON'T WALK" or upraised hand symbol.

The wider the street, the more time needed to cross and the less time available for the green light in the opposite direction. (* Four feet per second is a "rule of thumb." Other variable such as railroad preemption and/or a higher population of elderly pedestrians may effect the values used in this calculation.)

Cross Traffic: Like pedestrian crossing, enough time should be allocated to clear the waiting traffic on the cross street. The heavier the cross traffic, such as experienced near schools, businesses, and other heavy traffic generators, the more time needed to clear them through the intersection and the less time 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; left-turn signals are usually installed.

The amount of time for left-turning traffic also limits the time permitted for the "through" traffic flow in the opposite direction.
Each of the above factors limit the amount of time for the green light in the "coordinated" direction.

Two-Way Traffic Flow: Another thing that limits the amount of time for the green light in one direction is the need for "coordination" in the other direction as well. The distance between traffic signals and the speed of the traffic determine the way in which the green lights at the next traffic signal "line up."

If the spacing is not equal between traffic signals, the green lights may only "line up" well in one direction. When this happens, the green lights will normally "line up" better in the direction with the most traffic. The traffic in the other direction may have to stop.

Off-Peak Traffic Periods: Another reason that you may have to stop is that the traffic signals are not coordinated. During times when traffic is light, traffic signals often are allowed to run independently. Traffic signals are most often coordinated during the "peak" travel times when traffic is heaviest. These times are usually between 7:00-9:00 a.m. and 4:00-6:00 p.m.

Why do I have to wait so long on a side street?

Remember that 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 left-turn arrows for all directions and wide cross streets.

For that reason, the time period that is fixed for each traffic signal may be rather long. So, if you are waiting for a green light to cross the "coordinated" street where there are no left-turns arrows and very light traffic on the side street, chances are very good that you will feel like you are waiting for a very long time.

Actually, you should rarely have to wait any longer than about two minutes. This can sometimes seem like a very long time.