Traffic Signal Operation

How traffic signals work

The public's understanding of the function of traffic signals can improve driving habits by reducing speeding and associated traffic accidents. The more drivers know about the operation of traffic signals, the less they will be frustrated when waiting for a signal to change.

Why are traffic signals needed?

As traffic volumes increase beyond the capability of lesser controls such as a four-way stop, it may be necessary to install a traffic signal.

Traffic Signal Equipment

Traffic signals are more costly than is commonly realized, even though they represent a sound public investment when justified. A modern signal can cost up to $250,000. This money pays for:

  • A Traffic Signal controller
  • Signal Heads
  • Vehicle Detectors
  • Signal Poles and Supports

The Controller is the signal's brain. It consists of a computer that controls the selection and timing of traffic movements in accordance with the varying demands of traffic as registered with the controller unit by detectors.

Signal Faces are part of a signal head provided for controlling traffic in a single direction and consisting of one or more signal sections. These usually include solid red, yellow, and green lights and sometimes red, yellow and green turn arrow lights as well. The Signal Head can contain one or more signal faces.

Detectors are devices for indicating the passage or presence of vehicles. Older locations use loop detectors. These consist of wire loops placed in the pavement near the intersection. They are activated by the change of electrical inductance caused by a vehicle passing over or standing over the wire loop.

Newer signals use video detection. A camera points down at each approach of the intersection. Digitized areas are drawn within the travel lanes. A computer processor senses the passage of objects through these digitized areas causing the intersection to turn green or remain green.

Traffic Signal Preemption

The transfer of signal control to a special signal operation is called preemption. There are two common types of preemption, railroad and emergency vehicle.

  • Railroad preemption is initiated when a train passes over advance detectors located on the tracks ahead of the railroad crossing. The purpose of railroad preemption is to clear tracks of traffic that may be stopped on or near them by traffic signals.
  • Emergency vehicle preemption can be used for any authorized emergency vehicle, but normally only for fire engines. The purpose is to provide a green light for the emergency vehicle as soon as possible or to hold an existing green light. To obtain a green light, existing green lights, including pedestrian intervals, are abbreviated or skipped. After the yellow change interval, a green light is given to the approaching emergency vehicle. The type of signal preemption used for emergency vehicles is a modulated strobe light. A detector at the signal receives light modulated output from a vehicle mounted emitter.

Flashing Red

According to the Arizona Revised Statutes, when a red lens is illuminated with rapid intermittent red flashes, a driver shall stop before entering the crosswalk on the near side of the intersection. The driver may proceed subject to the rules applicable to making a stop at a four-way stop controlled intersection.

Flashing Yellow

When a yellow lens is illuminated with rapid intermittent yellow flashes, a driver may proceed through the intersection or past the signal only with caution.

Dark Signals

When a traffic signal has gone dark due to power failure, it is considered to function the same way as a four-way stop controlled intersection and a driver must stop before entering the intersection.

Emergency and pedestrian beacons are an exception to this rule. Both devices are clearly signed and remain dark until needed by an emergency vehicle or crossing pedestrian.

Signal Timing: How they "know" when to change from red to green.

Pre-timed traffic signals have pre-set time intervals for different times of the day including the morning, noon, and evening peak travel periods. These intersections automatically cycle to all movements regardless of the presence of traffic. Most of these signals are located in the downtown area.

Traffic actuated signals use approach detectors to monitor and assign the right of way on the basis of changing traffic demand.