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