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Traffic Signals

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1. How do traffic signals work?

There are three basic types of traffic signals:

Pre-timed: The signal serves each phase of traffic without sensing any traffic at the signal.

Semi Actuated: The main flow of traffic won’t sense any traffic, but the side street senses cars.

Fully Actuated: All phases of traffic sense cars and the signal cycles on demand. The controller is programmed for minimum and maximum green times. The green phase of traffic will terminate in two ways: The green phase served its maximum time, or the phase gapped out (excessive amounts of time between cars).

2. How does a traffic signal sense cars?

When a signal is installed, multiple 6' by 6' coils of wire are placed under the road surface. These wires are connected to the controller equipment. A small electrical signal is present on the wires and when a vehicle drives over the loops it changes the inductance value on the wire. This change of inductance is recognized as a vehicle presence and the controller acts accordingly. Vehicles should always stop at the white stop bar painted on the road. This is where the last loop in your lane is located. Usually there are multiple loops per lane in order to achieve good vehicle detection. Loops can also count vehicles.

3. Why do some lights stay green when there are no cars present?

If the signal was designed to operate as a pre-timed or semi actuated signal, it could rest green until the controller allows the next phase to go green.

Sometimes the loop wires become damaged and the signal controller will interpret the damaged wires as a vehicle call. This will hold the phase green for the maximum amount of seconds programmed into the controller.

If a pedestrian push button has been damaged or jammed so the button can't release, the signal will serve the pedestrian call every cycle. This will cause the signal to stay green while it serves the pedestrian call.

If the signal is running in a coordinated string of traffic signals, it will hold green in order to maintain its coordination with the other signals.

4. How does a fire truck change the lights to green?

Most fire engines in Missoula have a high frequency strobe light that is mounted on top of the engine. The controller equipment at the signal has a directional detector that sees the engine coming. The controller equipment safely clears any cross street traffic, pedestrians, and then gives him the green light. Once the engine has passed, the light will cycle normally.

5. Can I make a light change to green by flashing my headlights?

No. If you think you have done this, the light was in the process of changing anyway.

6. What happens when a train is near the signal?

The railroad equipment is linked to the traffic signal controller. When a train comes, their electronic equipment triggers our signal controller and the controller performs a preprogrammed sequence in order for the train and vehicles to pass safely.

7. How are traffic signals coordinated in a system?

The new traffic signal controllers are very sophisticated. They are based upon high-speed microprocessors that make thousands of logic calculations every second. Very accurate time clocks are integrated into the controllers functions. If all the clocks at each intersection are set the same, the controller knows when to coordinate each coordinated phase based upon the amount of time that has elapsed since the last midnight. This allows the signals to be coordinated together without connecting wires between them. Some signals are interconnected with wires due to other coordination requirements.

8. How do pedestrian lights and buttons work?

The walk light will only come on with its associated green light. When a pedestrian push button is pressed, (it only needs to be pressed once) a pedestrian call is placed in the controller memory. Depending on when the button was pressed in the signal cycle, you might have to wait a few seconds before you will get the walk light. There is a lot of confusion about the timing of the walk and don’t walk lights. The walk light only comes on for a short period of time. This short walk time is only meant to get you started across. The flashing don’t walk sign then starts flashing. This is the time used to finish walking across the street. If the don’t walk light is on solid (not flashing) you shouldn’t be in the street.

9. Why do I need to push a button to get a walk light?

Traffic lights have different requirements at almost every location. Some signals don’t have pedestrian push buttons. The signal serves the pedestrians every cycle. These are signals that have heavy pedestrian traffic so pushing the button isn’t needed. The signals that have occasional pedestrian usage need to have pedestrian buttons in place. Sometimes when a signal gets a pedestrian call, it takes a lot of time to serve the pedestrian. This is sometimes longer than the vehicle passage time would be, thus it can cause the vehicle efficiency to drop. The signal timing is very critical in most intersections and efficiency is very important to uniform traffic flow.

10. What new technologies are coming our way?

The rapid advancements of traffic signal technology are a great benefit to us. New ideas and options are available every day. Ideas are constantly being tested. Video detection is one of the hottest new issues today. The cost at this time is very high and many refinements need to be done. The loops that we put in the roadway for vehicle detection would be a thing of the past. Road construction and maintenance digging wouldn’t effect the signal operation. New red LED (light emitting diode) lights are being used in the Missoula area. This red light doesn’t use an incandescent lamp, but uses many LEDs. This allows the indication to last about 8-10 years. The normal red lamp would use 150 watts of power. The new LEDs use about 12 watts of power. Amber LEDs are not cost effective yet because of problems producing the correct colors in LEDs.