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1. How do traffic signals work?

There are three basic types of traffic signals: Pre-timed: The signal changes according to a predetermined schedule and does not sense the traffic at the signal. Semi Actuated: The main flow of traffic has the green light and changes only when the signal senses cars on the side streets. Fully Actuated: All phases of traffic sense cars, and the signal changes as needed. The controller is programmed for minimum and maximum green times. This type of light will stay green until either it has run its maximum time or there is a large amount of time between cars.

2. How does a traffic signal sense cars?

Loops:  This detection type involves multiple 6-foot by 6-foot wire coils (loops) installed under the road surface. When a vehicle drives over the loops, a vehicle detector is activated and sends a message to the traffic signal to change the signal accordingly. Loops are the oldest form of traffic detection and are no longer installed with new signals because they require drivers to stop in a specific area, don’t work well for motorcycles or bicycles, and are often destroyed by excavation or roadway deterioration.

Video detection cameras:   These cameras are mounted for each approach to an intersection, and when they detect a vehicle, the signal will change. Cameras were installed in Missoula from 2001 until 2012 but are no longer installed because they can be affected by the sun and other light glare, fog, smoke, and build up on the lens from blowing snow and dirt.    

Radar: The newest and best option for vehicle detection at this time is radar because it accurately detects all roadway users and is reliable.

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

The types of signals installed in Missoula vary, so some reasons for this include the following:

• Some signals give each signal phase a certain amount of time on green and will change only when that time expires.
• If the signal is running in coordination with other traffic signals on a corridor, it will hold green in order to maintain its coordination with the other signals.
• If a pedestrian pushes a signal button, the signal will stay green while it serves the pedestrian call.

4. How does an emergency vehicle, such as a fire truck or ambulance, change the signal to give them a green light in the direction they are traveling?

Most fire engines and ambulances have a coded infrared strobe mounted on top of the vehicle. When the strobe is activated, it is detected by a sensor at the signal that turns the signal green for the approaching emergency vehicle. Once the emergency vehicle has passed, the signal will return to the pattern it was running before.

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

No, flashing headlights will not cause a traffic signal to change.

6. What happens when a train approaches a traffic signal near a railroad crossing?

When a train approaches an intersection with a roadway, the railroad has equipment that triggers the traffic signal controller to run a pre-programmed railroad routine, which stops traffic, allows traffic to clear off of the tracks, and allows the train to continue along its route.

7. How are traffic signals coordinated in a system?

Traffic signal controllers are very sophisticated and use high-speed microprocessors that make thousands of calculations every second. Each coordinated intersection has a controller that is linked via a wireless network to a central server, which keeps all the traffic signals’ internal clocks synchronized. The coordinated signals run according to plans derived from vehicle counts. Typically, signals have four weekday plans—morning, off peak, noon, and evening—and one weekend plan. Coordinated traffic signals drop out of coordination in the evening when traffic volumes are lower.

However, in Missoula the seven downtown signals are fixed-time signals that run in coordination 24/7.

8. How do pedestrian buttons and walk signals work?

Once a pedestrian push button is pressed, a walk signal will come on with the associated green light for vehicles. Signal controllers handle these calls for service in sequence, so the walk signal will activate when it is that direction’s turn in the sequence. Just like a vehicle pulling up on a side street, pedestrians have to wait for the signal to cycle around to receive the walk light.

 The “walk” indication comes on for only a short period of time—usually 7 seconds—and is meant to get pedestrians started across the street. When the red hand (don’t walk) indication starts flashing, pedestrians are no longer allowed to leave the curb. However, if they are already in the crosswalk, they may continue crossing the street. Missoula’s signals have pedestrian countdown timers that work in conjunction with the red flashing hand indication to show pedestrians how much time they have before the solid red hand (don’t walk) indication will be displayed. Pedestrians should not be in the street when the solid red hand is displayed.

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

Traffic lights have different requirements at almost every location. Some of the older signals don’t have pedestrian push buttons, so the green light serves as the walk signal, serving pedestrians every cycle. The rationale was that these signals are in higher pedestrian use areas, so the pedestrians were served each cycle; therefore, the pedestrian buttons weren’t needed. The signals that have only occasional pedestrian usage need to have pedestrian buttons. Sometimes when a signal gets a pedestrian call, it takes a while 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.

All the newer signals have ped push buttons, and the very newest have Accessible Pedestrian Signal (APS) push buttons. The APS push buttons have both audible and tactile features for visually impaired pedestrians. Eventually, all signals will have APS pushbuttons.

10. What new technologies are coming our way?

The next evolution in traffic signals is adaptive signal control technology, which is a system of coordinated signals that communicate with each other and vary signal timing, cycle lengths, and other parameters by real-time traffic volumes instead of the conventional time of day plans. This will allow greater efficiency by responding to actual demand, rather than predicted demand. It may benefit pedestrians as well as drivers by serving them sooner, especially when the main street traffic is low.

The City of Missoula and other Montana communities are working with the Montana Dept. of Transportation to identify corridors that may benefit from adaptive signal control and to put together projects to build these systems. Much of the technology needed for this is already in place, but additional equipment and many more traffic detection devices would be required.