IV. Local Controller Parameters
It is useful to identify traffic signal timing parameters as being in one of two categories: local intersection timing parameters and coordinated operation timing parameters. The phase minimum or yellow change times are examples of local intersection timing parameters. The intersection cycle length and offset are examples of coordination timing parameters. There are far more local intersection timing parameters than coordination timing parameters. In fact, most systems define only three coordination timing parameters: cycle length, offset, and split. The following section of this report describes methods that can be used to set the local intersection timing parameters, while the next section deals with the coordinated operation parameters.
The basic timing parameters are essentially the same for all actuated controllers. There are subtle differences between different software implementations; for example, the NEMA controllers define the force-off function as a “per ring” function, while other implementations define the force-off function as a “per phase” function. This distinction has little importance to the traffic engineer who is responsible for developing new Traffic Signal Plans. These differences, however, are very important when the results of a signal timing optimization process are implemented in a particular controller. Many newer controllers automatically calculate the force-off setting once the engineer defines the split.
Because most jurisdictions use NEMA TS-1 and TS-2 or Model 170 actuated controllers, the primary emphasis has been placed on timing actuated controllers. It is important to recognize, however, that many jurisdictions use Pre-timed controllers. Most of the principles noted in this section can be directly applied to Pre-timed controllers.
For the current status on signal timing, the reader is directed to Signal Timing Practices and Procedures: State of the Practice (Item IR-116) published by the Institute of Transportation Engineers (ITE). This report documents the current state-of-the-practice in traffic signal timing. It was prepared as part of a continuing program developed by FHWA and ITE to place increased emphasis on the quality of U.S. traffic signal timing.
Some of the basic principles of timing the green interval in a traffic actuated controller unit are as follows:
Each phase must have a minimum green time so that a stopped vehicle that receives a green signal has enough time to get started and partially cross the intersection before the yellow signal appears. This time is sometimes called the initial portion of the green interval.
Each vehicle requires enough green time to travel from the detector to the intersection. This is called passage time, vehicle extension, or gap. Gap refers to the distance between vehicles as well as the time between vehicles. Each successive vehicle actuation, therefore, increases the phase green time. With no opposing calls, the controller rests. Extensions continue to be timed, but with no effect on the green interval.
There must be a maximum time that the green interval can be extended if opposing cars are waiting; this is called the Maximum Green or extension limit.
Once an actuation is received from an opposing phase, the Maximum Green timer begins. The current phase will hold the green until the time between actuations is greater than the preset unit extension or gap. When a gap is detected, the yellow change interval will begin and the controller will transition to the next phase in sequence with demand. This is called termination by gap or gap-out.
An actuation from another phase received during any portion of the green interval also starts the Maximum Green timing circuit. This is also called the extension limit. Even if actuations are close enough in time to prevent gap termination, the maximum limit will terminate the green interval when the preset maximum expires. This is called termination by maximum green or max-out.
All actuated controllers support these five basic, phase-timing parameters: minimum green, extension, maximum green, yellow and red.
The minimum green is the first timed portion of the green interval. The duration of the minimum green is generally based on the number of vehicles that can be in queue between the upstream phase detector and the stop line. In general, the phase duration must be no shorter than some absolute minimum time, such as 5 to 10 seconds. If pedestrians may be crossing with this phase, their crossing time must also be considered and included in the minimum phase length.
The term, minimum green, is called “minimum initial” on some controllers. There is a subtle difference. The minimum green defines the duration of the green when there are no extensions. When the controller uses a minimum initial, the minimum green duration is equal to the sum of the minimum initial and one extension.
The minimum green time is the minimum assured green that will be displayed. It is established to allow vehicles that are stopped between the detector on the approach and the stop line to get started and move into the intersection. Therefore, timing of this interval depends on the location of the detector and the number of vehicles that can be stored between the detector and the stop line.
Consideration must also be given to pedestrian timing. When there are no pedestrian provisions (indications or pushbuttons), the minimum assured green must be equal to the minimum pedestrian timing (walk + pedestrian clearance).
One method that can be used to calculate the minimum green is:
Minimum Green = 5 + 2n
“n” is the number of vehicles that can be stored between the stop line and the far detector in one lane. This is determined by dividing the distance (in feet) between the stop line and the detector by 25 since 25 is the average vehicle length plus stopped-headway in feet.
When stop-line detection is used, the minimum green can be quite short, say 5 to 10 seconds. These very short minimums can be used on phases where there are no pedestrian movements, and on phases where there are actuated pedestrian signals. These extremely short minimums should never be used where there is the possibility of pedestrians crossing with the green display.
The extension (passage) parameter extends the green interval for each vehicle actuation up to the maximum green. It begins timing when the vehicle actuation is removed; that is, when the vehicle passes over the detector. This extension period is subject to termination by the maximum extension timer or a force-off.
The actual passage time parameter (vehicle extension or gap time) is the time that the phase will be extended for each actuation. This setting is the number of seconds required for a vehicle moving at the approach speed to travel from the detector to the stop line. The passage time serves two purposes: (1) it is the passage time from the detector to the stop line, and (2) it is the allowable time gap between actuations that will cause the green to remain on that approach. As long as vehicle detections come at shorter intervals than the passage time (allowable gap), the green will be retained on that phase until the maximum expires or it is forced-off.
If the passage interval is too short, quick stops may result as well as terminating the green before the vehicular movement has been adequately served. If the passage interval is set too long, excessive delays will result as well as safety problems due to improperly timed last vehicle intervals.
Passage Time is calculated as follows:
Passage Time = D / S
D is the distance from the stop line to the detector in feet
S is the speed on the approach in feet per second.
This time setting defines the maximum length of time that a phase can be green in the presence of a conflicting call. If there is no conflicting call, it will be reset until an opposing call occurs.
The maximum green timer is normally inhibited during coordinated operation when the phase maximum time is determined by the phase force-offs. For most actuated phases, the maximum green should be considered a safety constraint, something to force the phase to end in the face of continuous demand. There is no accepted practice to determine the optimum setting. Some engineers feel that all phases should have similar limits like: 120 seconds for the primary through phases, 90 seconds for the secondary through phases, and 60 seconds for the left-turn phases. Others set the phase maximums proportional to demand during a critical period. For example, one could calculate the phase splits using the Critical Movement method then set the maximum green at 50 percent higher than the phase splits.
The yellow interval follows the green interval at the end of each phase. The yellow interval is also referred to as the “change” interval and controls the duration of the yellow display for that phase. The phase change interval timing advises drivers that their phase has expired and they should: (1) come to a safe stop prior to the stop line, or (2) proceed through the intersection if they are too near the intersection to stop.
The following equation is generally used to determine the proper change interval:
Yellow Time = t + S / (2a +-64.4 g)
t is the perception/reaction time of the driver in seconds (typically 1.0 second).
S is the speed on the approach in feet per second.
a is the deceleration rate in feet per second (typically taken as 10 feet per second squared).
g = approach grade, percent of grade divided by 100 (add for up-grade and subtract for downgrade).
The red clearance interval (also known as the all-red interval) follows the yellow interval of each phase. It must expire before the next phase in sequence can begin. It is normally one to two seconds, but on slower speed approaches, it is not unusual to use a very short duration of 0.0 to 0.5 seconds since the Yellow time of 3.0 to 4.0 seconds provides sufficient time to meet both the change and the clearance requirements.
Red Time = (W + L) / S
W is the Width of intersection in feet.
L is the length of vehicle in feet (typically taken as 20 feet).
S is the speed on the approach in feet per second.
There are two pedestrian timing parameters: Walk and Flashing Don’t Walk (pedestrian clearance). Where pedestrian movements regularly occur, pedestrians should be provided with sufficient time to cross the roadway (MUTCD section 4D.03). In other words, unless the pedestrian movement is actuated, the time required for pedestrians to safely cross the road (the sum of the Walk and Don’t Walk times) will dictate the minimum green time for that phase. When pedestrian pushbutton (actuation) is provided, then the normal vehicle minimums would be used in the absence of a pedestrian actuation. Notice that it is possible to have a signal design that uses pedestrian pushbutton detection without using pedestrian Walk and Don’t Walk displays.
The Walk parameter controls the length of time that the walk signal is displayed. Under normal conditions, the walk interval is 4 to 7 seconds. This allows pedestrians to have adequate opportunity to leave the curb before the clearance interval is shown. Under special circumstances, such as at a school crossing with numerous pedestrians, walk times may exceed 7 seconds. Research indicated that pedestrian queues of 24 or more people can be accommodated in 7 seconds.
The pedestrian clearance parameter controls the duration that the Flashing Don’t Walk is displayed. This is the time required for a pedestrian crossing in the crosswalk to leave the curb and travel out of the traveled way (MUTCD section 4E.02) before opposing vehicles receive a green indication.
The Flashing Don’t Walk (pedestrian clearance) is calculated as follows:
Flashing Don’t Walk = W / SP
W is the walking (crossing) distance in feet from curb to curb.
SP is the average walking speed in feet per second (typically 3.5 to 4 feet per second).
In the 1950s the Automatic Signal Company introduced two new and complex traffic signal controllers, the Model 1022 and the Model 1033. These controllers had far more timing circuits than any controllers used before or since. Although all of the timing parameters were well-founded in theory; in practice, many did not actually improve the performance of the intersection. There were several reasons for this experience: it was not possible to determine which of the many timers actually caused a particular phase to terminate; the analog timing circuit technology available at the time was subject to variations based on temperature and humidity; and like today, few engineers had the time necessary to fine-tune the operation. Of the many features of these innovative controllers, two features (variable initial and gap reduction) have evolved and are typically available with today’s controllers. These two features are explained below.
To successfully employ the volume density features of a controller, it is necessary to have phase detection farther upstream than that which is normal. This enables the controller to sample traffic on the approach with a minimum impact from a standing queue. Detection 300 to 400 feet upstream from the stop line is typical with volume density operation. With detection this far back, the Minimum Green would be quite long—35 seconds with the detector back 400 feet. Since a Minimum Green this long would result in very sluggish operation when the demand is light, two other timing circuits are used to mitigate these impacts: Added Initial and Maximum Initial.
Added Initial—This interval times concurrently with the minimum green interval. The value is the amount of time that is added when each vehicle actuation is received during the initial period. The actual minimum green time used by the controller is the greater of the minimum green or the added initial sum. The added initial cannot exceed the maximum initial. The added initial allows the duration of the minimum to vary between the actual minimum and the time required to discharge the maximum queue that can be stored between the stop line and the detector.
Maximum Initial—This is the maximum period of time for which the added initial can extend the initial green period. The maximum initial cannot be less than the minimum green.
Gap reduction is the other legacy concept that is used on the modern controller. This parameter is used to allow the “gap seeking” logic of the actuated controller to identify a smaller gap in traffic to justify terminating the phase earlier than that which is implied by the passage time. Gap reduction is a means of reducing the passage time or gap on the basis of the time that opposing vehicles have waited. In effect, it benefits the waiting vehicles by reducing the time allowed between vehicles arriving on the green phase before that phase is terminated. There are three timing parameters associated with the gap reduction feature: Time-Before-Reduction, Time-to-Reduce, and Minimum Gap.
Time-Before-Reduction—This period begins when the phase is green and there is a serviceable call on a conflicting phase. When this period is completed, the linear reduction of the passage time begins.
Time-To-Reduce—This period begins when the Time-Before-Reduction ends and controls the linear rate of reduction until the Gap is reduced to the Minimum Gap.
Minimum Gap—Like the passage time, this parameter extends the green interval by the minimum gap time for each vehicle actuation up to the maximum green. It begins timing when the vehicle actuation is removed. This extension period is subject to termination by the maximum green or a force off.
There are 12 controller timing parameters that are commonly used:
WALK—Establishes the length of the WALK interval.
PED CLEARANCE—Establishes the length of flashing DON’T WALK interval.
MINIMUM GREEN—Establishes the length of initial state of green interval.
PASSAGE TIME—Establishes the increment of right-of-way (green) time extension for each vehicle actuation during the green interval.
MAXIMUM GREEN—Establishes the maximum limit to which the green interval can be extended on a phase in the presence of a serviceable demand on a conflicting phase.
ADDED INITIAL—Density feature. Establishes number of seconds by which each vehicle (actuation) increases the initial state of green during non-green time on the phase
MAXIMUM INITIAL—Density feature. Establishes the maximum limit to which the initial interval can be extended on a phase.
TIME B4 REDUCTION—Density feature. Establishes a preset time before the allowed gap (Passage Time) begins to reduce.
TIME TO REDUCE—Density feature. Establishes time in which the allowed gap is reduced from passage time to minimum gap, after the time before reduction has expired.
MINIMUM GAP—Density feature. Establishes minimum value to which allowed gap between actuations on phase with green can be reduced upon expiration of time to reduce.
YELLOW CHANGE—Establishes the length of yellow interval following the green interval.
RED CLEARANCE—Establishes the length of red clearance interval following the yellow interval.