All about Traffic Signals
By Jim Bloodgood, P.E., Snohomish County Traffic Engineer
Q: Why do we need traffic signals?
A: As traffic volumes increase on roads beyond the capacity of lesser intersection controls such as four-way stops, it may be necessary to install a traffic signal.
Traffic signals offer maximum control at intersections. They are designed to ensure a safe, orderly flow of traffic, increase the traffic handling capacity, provide safety for motorists or pedestrians while crossing a busy intersection, and help lessen the severity of collisions entering intersections.
Q: Do signals prevent collisions?
A: No. While signals can greatly reduce the number of right-angle collisions at an intersection, they also can cause a significant increase in rear-end collisions. Normally, traffic engineers will accept an increase in rear-end collisions for a decrease in the more-severe right-angle collisions; however, when there is no right-angle collision problem at an intersection and a signal is not needed for traffic control, there is no safety benefit and the installation of traffic signals can actually cause a deterioration in the overall safety at the intersection.
Q: How do traffic signals work?
A: They permit conflicting streams of traffic to share the same intersection by stopping some and letting others pass through. They assign right-of-way and relay messages to motorists and pedestrians regarding what to do and what not to do by displaying red, yellow and green signals, turn arrows or other indicators.
Q: How much do traffic signals cost?
A: Traffic signals are more costly than most people realize. A modern signalized intersection (which can consist of numerous individual “signal heads”) can easily cost $250,000-$500,000 or more, depending on the amount of road work that must be done in conjunction with the signal installation. The money pays for a traffic signal control panel, signal heads, vehicle detectors, signal poles and supports. Maintenance and operation costs tend to run about $1,200 per year.
Q: Who decides when and where to install a traffic signal? How can I get one installed?
A: Before installing a signal at an intersection, a traffic engineer must evaluate the amount and flow of vehicular and pedestrian traffic; special conditions such as hills and curves; future road construction plans in the area; and the collision history of the intersection.
Q: Whom do I contact if a traffic signal is dark or malfunctioning?
A: It depends on where the signal is. If it’s within a city limits, you need to contact that city’s public works or street department. If it’s in unincorporated areas, contact the county. And if it’s on a state or federal highway, contact the Washington State Department of Transportation.
Wherever it is, if the signal is dark, always treat the intersection as a four-way stop.
Q: How have traffic signals changed over the years?
A: Early signals were basically just incandescent light bulbs connected to a clock that made them turn on and off at regular intervals. The next step was a timer that could be adjusted to operate differently at different times of day.
Modern signals are much more “intelligent” than that. First of all, many signals use Light Emitting Diodes (LED), which have greater visibility and dependability than the old bulbs. LEDs cost more to purchase, but they last a lot longer and have lower maintenance and operating costs.
Today’ signals are mostly inter-connected in a computerized system so they can work together in a controlled, coordinated way. Video detectors (overhead cameras) or loop detectors (buried in the pavement) let an individual signal controller know what vehicles are passing or present at that intersection, so the signal can automatically change to accommodate prevailing traffic.
Also, modern signals have special preemption functions to give priority to trains, emergency vehicles and buses.