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Some stops are simply unavoidable with the street system. For example, not all signals are uniformly spaced. Different intersections have different traffic demands, so cycle lengths must vary. And we must keep traffic at or near the speed limit.
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Coordinated traffic signals provide the following benefits to motorists:
It is impossible to time the signals so that no driver hits a red light.
The traffic signals are coordinated to help traffic flow through a series of signals and avoid stops. This is called signal progression.
Ideally, Traffic Signal Progression ("timed" signals) is a process by which the signals along major roads are programmed to permit cars traveling at the posted speed to travel uninterrupted through a series of green lights. Signal progression has been used in major cities across the United States for decades.
While major roads are already programmed for signal progression, there are factors that undermine the success of such a system, including accidents, an excessive number of intersecting driveways between lights, gridlock (back-ups that block an intersection), and interruptions due to heavy pedestrian signal use. In addition, drivers who speed, change lanes too frequently, or drive too slowly will arrive at the next signal too early or too late and will disrupt the signal progression for other drivers.
The major disadvantage of programmed signal technology is that it lacks flexibility - it can't sense road conditions and adapt accordingly. In the event of a car accident, signal timing can't detect an accident or expedite the delivery of emergency or law enforcement services, nor can signals on timers advise motorists about alternative routes or adjust timing to correct delays.
On two-way streets, coordination is much more complex. Three variables must be taken into account:
Nonstop traffic flow can only be provided where exact relationships exist between these three variables.
Intersection signals are coordinated, or synchronized with each other to reduce stops and delay for the major traffic movements. Coordinating signals requires that all signals be programmed with a common cycle length, which is the amount of time it takes a signal to sequence through all traffic movements at one time. The quality of movement through a series of traffic signals depends on the spacing between signals, the speed of traffic, the cycle length, and the amount of traffic. Signals along main arterials are generally coordinated with each other during the day when there are heavy traffic flows. It is often not possible to progress traffic in both directions because of poor spacing between traffic signals. Sometimes it is necessary to choose one direction to progress. When two-way progression is not possible, the City often uses computerized traffic modeling to find coordinated timing plans that decrease the total delay and stops for all users of the system. Traffic turning onto or off of a side street is generally not progressed, and turning vehicles can usually expect to stop at the next signal.
There are many issues that may affect the coordination of traffic signals. It is easy to coordinate signals that are on one-way streets to provide for very efficient movement of traffic. That is why you will see one-way streets in many downtown areas - it is the most efficient way to move large amounts of traffic with relatively few lanes. Coordinating signals on two-way streets where the signals are irregularly spaced is a much more difficult challenge. In some cases, it becomes physically impossible to provide good coordination simply due to the spacing of the intersections and the speed which people are driving.
At most traffic signals several different timing plans are used throughout the day to account for varying levels of traffic demand. The length of the wait depends on the signal cycle length and the amount of traffic. In general, a longer cycle length increases the amount of vehicles that can be moved through an intersection (capacity). Increasing cycle lengths also increases driver delay. Cycle lengths range from 60 seconds to 90 seconds, depending on the size of the intersections and the amount of traffic. Cycle lengths must be longer at larger intersections to serve the greater number of separate traffic movements during the timing sequence, to accommodate much longer pedestrian crossing times, and to accommodate higher volumes of 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 progression. 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 and on weekends.
The amount of green time programmed for each movement at a signal varies by time of day. Sometimes there is more traffic at a signal than the signal can handle, and the signal is over its capacity. In these situations, the signals are timed to equalize delays for conflicting movements. At other times green time can be moved from one movement to a conflicting movement, realizing that improving one movement hurts another. Increasing green for one movement requires decreasing the amount of green time for another movement.