Traffic signals | WSDOT - WA.gov

Learn more about how traffic signals operate, and how and when they are installed.

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Traffic signals are a vital tool used to manage traffic interactions at an intersection. For the most efficient operations, a traffic signal system must be monitored and adjusted periodically to serve changing traffic patterns.

A properly optimized traffic signal system has multiple goals:

• Reducing delay for all users.
• Improving safety.
• Reducing air pollution and use of fossil fuels (less time idling).

Traffic signal timing

Traffic signal timing is managed by a special computer called a traffic signal controller. This controller is programmed with the time needed for each signal phase (green and walk times) and clearance times (red, yellow, and don't walk times). Optimum timing is when all users are subjected to the least amount of delay possible. With the different volumes and modes of travel moving through a signalized intersection, this can be particularly difficult to achieve.

At an existing system, traffic engineers collect data including traffic patterns, speeds, and volumes. This data is analyzed and models are generated so that timing plans can be updated and implemented that fit the current conditions.

Vehicle signal times

For vehicles, green times are normally set to allow the greatest amount of traffic through, with the least amount of delay for other conflicting directions of travel. Yellow times are set in an effort to allow drivers enough time to stop safely at the onset of yellow. Red times are set to allow the intersection to fully clear before a new direction of travel enters the intersection.

Pedestrian signal times

Pedestrian walk times are set similar to vehicle green times. Pedestrian clearance (flashing "don't walk") are normally set based on walking speed, as described in MUTCD Section 4E.06. Wider crossings result in longer pedestrian clearance times. The timing of the "don't walk" has a minimum required before conflicting vehicle traffic may be released, and is also described in MUTCD Section 4E.06.

Overall cycle time

The overall cycle time, or cycle length, for a traffic signal system is the time it takes from the start of green for one direction until the next start of green for that same direction. The cycle length has to include all of the green, yellow, red, walk, flashing don't walk, and don't walk times for every direction served, although some of these overlap (such as opposing through traffic). Rarely is the cycle length longer than 120 seconds (2 minutes), and most traffic engineers strive to keep the cycle length as short as possible. If there is no traffic on a cross street, a signal system may "rest" in green for the major roadway, where the signal "pauses" in the middle of the cycle.

Coordinated signal systems

When multiple traffic signals are close together on a highway, they are normally interconnected and are often operated as a coordinated system. Coordinated Signal Systems are probably the most misunderstood signal system networks. A coordinated signal system network does not mean that a driver on the main roadway will get green lights all the way through the network. But the system is set up to get drivers through the entire network as quickly as possible.

There are times and locations where this can mean sequential green lights through the whole system, but that is not always the case. Traffic volumes, distances between traffic signals, sizes of intersections, pedestrian activity, and other factors may not allow for truly sequential green lights.

Many coordinated signal systems are not operated in coordination all the time, but instead only during peak commute hours. This is why networked systems may operate differently depending on the time of day, which may mean more or fewer stops for users.

Coordinated systems can also cause more headaches for side road users. Coordinated signal timing is typically based on the largest and most complex intersection in the network. This can result in longer than needed times at smaller intersections. If you find yourself waiting at a side street with little or no conflicting traffic, and it's in a coordinated system, this may be why. However, as with general cycle length, coordinated systems are normally set up so that no system in the network has a cycle length greater than 240 seconds (4 minutes), and are as short as possible.

Accessible pedestrian signals

Accessible Pedestrian Signals (APS) are traffic signal system devices which combine a pedestrian detector &#; commonly a pushbutton &#; with pedestrian signal indications in non-visual formats. These non-visual indications are both audible and tactile.

APS speech messages

We require that all APS units used in WSDOT owned and operated traffic signals include speech messages, even if they may not be used at an intersection. In order to provide a consistent message throughout the state, speech messages must follow the APS Sample Message Table (PDF 456KB). We also require that all APS units at an intersection operate in the same way, where all use speech messages or all use the "rapid tick" indications.

Installing a traffic signal

Installation of a traffic signal requires a significant amount of investigation and evaluation. Traffic signals are used to solve specific traffic problems, and may potentially cause new ones - particularly when installed in inappropriate locations. Traffic signals are generally most beneficial when trying to provide breaks in major roadway traffic for crossing movements. To be truly effective, any traffic control device must be perceived as reasonable by users.

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Evaluation

Most states follow the MUTCD Warrant System (MUTCD Section 4C) to determine if a traffic signal should even be considered at an intersection. To make these evaluations, traffic data including volumes, speeds, and collision data is collected and analyzed. In some cases, a lesser form of control, such as stop or yield signs, may be tried and proven ineffective before a signal is considered.

Angle collisions, such as "T-bone" type collisions, and pedestrian collisions are common factors for installing a traffic signal. Traffic signals can reduce these types of collisions, but rear-end collisions typically increase after a traffic signal is installed. The trade-off is that these rear-end collisions generally are less severe than the higher-speed angle collisions.

Installation of a traffic signal at an inappropriate location may also have the unintended consequence of diverting drivers to alternate routes. These alternate routes may not be designed for increased or faster traffic, such as residential neighborhoods where drivers may not abide by lower speed limits, putting residents and others in greater danger.

As part of evaluating an intersection for a traffic signal, we will normally consider other possible control alternatives, such as roundabouts <need new link>. We make an effort to apply the most appropriate solution to intersection management, in order to keep traffic moving safely and smoothly.

Cost

Traffic signals are expensive systems. A typical basic traffic signal system will cost around $250,000 just for the signal system installation. If this is the entire construction project, this cost will be closer to $500,000 or more, as things like engineering, work zone traffic control, and supporting construction work is included in the project. Traffic signal systems also have ongoing power bills and maintenance costs for technicians and engineers to keep the system operating safely and effectively, which is roughly $8,000 per year. That works out to an additional $200,000 to keep a signal system operating for 25 years.

Requesting a traffic signal

If you think a traffic signal is needed at an intersection, contact the agency that manages the intersection and they can discuss the location with you.

For intersections on state highways, even inside cities, contact WSDOT. If the intersection is in an area where the state highway signals are managed by a city, we will get you to the appropriate agency.

For other intersections, contact the City Public Works Department for intersections within city limits, and the County Public Works Department for intersections outside city limits. You can also discuss with them other possible improvements to the intersection which may be enough to resolve your particular concern.

What is the purpose of a traffic signal?
Traffic signals are electronically operated traffic control devices which alternately direct traffic to stop and to proceed. Traffic signals are designed to ensure an orderly flow of traffic, provide an opportunity for pedestrians or vehicles to cross an intersection and help to reduce the number of conflicts between vehicles entering intersections from different directions.

How do traffic signals work?
Traffic responsive controllers change the lights according to the amount of traffic in each direction. These controllers use sensors (inductive loops in the roadway) to detect the number of vehicles and automatically adjust the length of the green time to allow as many vehicles as possible through the intersection before responding to the presence of vehicles on another approach. Although these types of traffic controllers have been in use for many years, a new generation of microcomputer traffic controllers makes the signalized intersection much more efficient, thereby reducing time-consuming delays.

How does FDOT decide whether a traffic signal should be installed on a State Highway?
The Department follows federal guidelines (Manual on Uniform Traffic Control Devices - MUTCD [This link opens a new browser window]) that establish minimum conditions under which a signal installation should be considered. From these federal guidelines, the Department has developed the Manual on Uniform Traffic Studies (MUTS). Both the MUTCD and MUTS manuals provide a process for the traffic engineer to follow while investigating conditions and circumstances regarding the installation of a new traffic signal or improve the operation of an existing traffic signal. They contain criteria (warrants) which are used to define the need for, and appropriateness of a particular traffic control device. These warrants are usually expressed in the form of numerical requirements such as the volume of vehicular or pedestrian traffic. Warrants should be viewed as guidelines, not as absolute values. However, if no warrants are met, a signal will not be installed. Satisfaction of a warrant is not a guarantee that the signal is needed. The warrant analysis process is just one of the tools to be used in determining if a traffic signal is necessary. Engineering judgment should be exercised in making the final determination.

Why does it take so long to get a traffic signal installed once it is approved?
After a traffic engineering study has indicated the need for a signal and the necessary approvals have been obtained, it must be determined how the signal will be funded and who will do the installation. If the signal is to be installed by a Contractor as a signal project, then it is added to a list of statewide locations which are competing for funds. These funds must be allocated well in advance which results in projects often "waiting" on the list for several years continually competing for funds against other projects which may have a greater need. Signals can also be installed by local governments in a joint effort with the Department of Transportation.

Will traffic signals reduce intersection crashes?
Traffic signals do not prevent crashes. Certain types of crashes can be reduced in number or severity by the installation of a signal, while other types will increase. Where signals are used unnecessarily, the most common results are an increase in total crashes, especially rear-end collisions. Traffic signals are not an answer for every problem intersection. A signal placed at a wrong location can contribute not only to rear-end collisions, but excessive delays, unnecessary travel on alternate routes and more congested traffic flow.

What does the red arrow mean?

A red RIGHT arrow means that you must come to a complete stop at the marked stop line or before moving into the crosswalk or intersection. After stopping, you may turn RIGHT on the red arrow at most intersections if the way is clear. Some intersections display a "NO TURN ON RED" sign, which you must obey.

A red LEFT turn arrow means that you must come to a complete stop at the marked stop line or before moving into the crosswalk or intersection, and shall remain stopped until a signal indication to proceed is shown. After stopping, the motorist facing a red LEFT turn arrow or red circular signal indication is permitted to enter the intersection to turn left from a one-way street onto a one-way street with traffic moving to the left except when a " NO TURN ON RED" sign is displayed.

How do I request a traffic signal study on a State Highway?
The District Traffic Operations Offices respond to written letters from the public, civic organizations, businesses, etc. requesting the need for a traffic signal to be installed or modified. In some cases, when the request involves private properties rather than public streets, it may be necessary for either the developer or property owner to hire a consultant to conduct the traffic signal study which must be submitted to the appropriate district for review.

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