Usually one of the following three things are occurring;
(a) there is too much traffic,
(b) someone is not paying attention, or
(c) a pre-emption.
Example 1: There is too much traffic. All roadways have a certain capacity and once this is exceeded, the traffic signal cannot clear each movement as is normally expected. During peak traffic hours traffic continuously approaches all directions at some signals during rush hour. All of the movements are given a generous amount of time but it is never enough to clear all of the vehicles in any/or some directions. If any of the movements at that intersection are delayed any longer than they already are, the backup grows exponentially.
Example 2: Someone is not paying attention. You are the fourth vehicle in line waiting to turn left. Normally, the green arrow has enough time to clear all four vehicles but this time the driver of the vehicle in front sped up after initially not realizing the light was green and made it through the yellow leaving you stuck until the next light.
Answer: The driver of the last vehicle that made it through the light was not paying attention causing too much time to elapse between vehicles passing over the detection zone. The signal sensed no more vehicles so it turned yellow.
The signal cycles even when no vehicles are present.
Every signalized intersection has a designated major street or movement, usually the through movement of the major street. All other movements are actuated or on demand. In order to have actuation, some sort of vehicle detection is necessary.
In Gilbert, the most common form of vehicle detection is a magnetic induction loop connected to a detector. This is simply a wire loop embedded into the pavement of each lane at the stop bar that extends back 20, 40 or 60 feet. When a vehicle is over the loop, the signal for that movement is "triggered" or called. Once a green light appears, the signal will stay green as long as it needs up to a set maximum amount of time. If only one or two vehicles are present then the green may last for 10 seconds or so. If 15 vehicles are present then it may stay green for 30 seconds or so. If no vehicles are present, the signal will not cycle to that movement.
Unfortunately, asphalt pavement is relatively soft and loops are very fragile so they often fail. When this occurs, the signal is set to cycle to that movement all of the time for an amount of time consistent with the amount needed during heaviest traffic.
Some signal intersections have been changing to use non-pavement invasive types of vehicle detection - video detection.
Video detection works by continuously taking a "picture" of an approach lane then comparing it to the previous picture. As long as the picture is continuously different than its previous one, the signal will stay green once it begins.
There is not enough pedestrian time to cross the street! The pedestrian signal does not work.
Every pedestrian signal has a WALK followed by a flashing DON’T WALK. Frequently, citizens will report a signal that does not allow enough time to cross the street on the WALK.
The WALK signal means to begin crossing the street and watch out for right turn on red motorists (although the pedestrian legally has the right-of-way, many motorists are not watching for pedestrians). A flashing DON’T WALK means that if one has already begun crossing, continue! The flashing DON’T WALK provides enough time for pedestrians to safely cross the street (MUTCD Traffic Manual using a walking rate of 4 feet per second). Flashing DON’T WALK also means if one has not begun to cross the street, then wait until the next WALK appears after pushing the pedestrian button.
Example: The walk light does not appear or is too short to cross the street.
Answer: First of all, a pedestrian WALK signal will not appear until a certain point during a signal cycle. Pedestrian signals do not make a signal appear any quicker than a concurrent vehicle movement. Once it appears, a WALK of at least 4 seconds indicates to the pedestrian to begin walking; then a flashing DON’T WALK occurs. This means if the pedestrian has begun, complete the crossing.
Why does the light take so long to change?
Traffic Signals that are in a coordinated mode are confined to a cycle length, which is governed by the cycle of a nearby major intersection. All signals along an arterial must have a common cycle length in order to achieve proper progression and burning less fuel.
Within that cycle length, a block of time is allocated to each movement. Each movement can appear only at a certain point in the cycle; once that has occurred, the movement cannot appear again until the next cycle. If a movement does not need all of its allocated time, the unused time becomes available to the next movement; this continues until all of the unused time, if any remains, ultimately is inherited by the main street movement where the cycle “zeros” itself out or begins again.
Cycle lengths vary depending on the time of day. During the AM and PM rush hours, signals have their longest cycle lengths because the major roads must accommodate the greatest amount of traffic. In the midday the cycle lengths are slightly shorter and the shortest cycles typically occur at night.