Traffic Signal Operations

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This page provides a brief overview of traffic signal operations concepts. Information on how to enter traffic signal timing inputs into SwashSim can be found here.

Signal Operation Modes

There are three main modes of operation for traffic Signals:
 * Pretimed
 * Semi-Actuated
 * Fully Actuated

Pretimed signals do not have detectors to actuate phases based on vehicle presence. Instead they use a fixed amount of time for each phase within the cycle. Pretimed signals can be operated in coordination with other signals or they can be operated in standalone mode, referred to as free mode. Semi-actuated signals typically use detectors on the minor street approaches. The major street phases are set to run for a maximum time, while the minor street phases will rely on vehicle detection to determine maximum phase times. This signal mode may or not be coordinated. If it is coordinated, additional logic is applied to be sure that green time is returned from the minor street to the major street at the right time to maintain the coordination band.

Fully-actuated signals have vehicle detectors for all active phases. Phase timing will vary based on vehicle actuations and cycle lengths will usually vary from one cycle to the next. Thus, this mode is usually reserved for "isolated" signals (i.e., signals not very close to other signals) or situations where coordination is not feasible (e.g., frequent signal pre-emption overrides from emergency vehicles).

Basic Signal Timings

Min Green This parameter represents the minimum amount of time that a green signal indication will be displayed for a movement. Minimum green is used to allow drivers to react to the start of the green interval and meet driver expectancy. A minimum green that is too long may result in wasted time at the intersection; one that is too short may violate driver expectation or (in some cases) pedestrian safety.

Max Green This parameter represents the maximum amount of time that a green signal indication can be displayed. Setting a max green time can help guard against long green times due to continuous demand or broken detectors. This helps keep cycle lengths to a fixed amount of time. A maximum green that is too long may result in wasted time at the intersection. If its value is too short, then the capacity of the intersection may be inadequate for the traffic demand, which will result in some vehicles being unserved at the end of the green interval.

Yellow Change Interval The purpose of the yellow change interval is to provide a warning to drivers about an impending change in the right-of-way assignment. The Institute of Transportation Engineers (ITE) formula is a common standard that is used to calculate yellow change interval values. The value for the yellow change interval generally should not be not less than 3.4 seconds and not longer than 6 seconds, as specified in FDOT standards.

For Approach Grades other than 0% use ITE formula:

$$Y=t+\frac{v}{2(a+gG)}$$

Where:
 * Y = length of yellow interval, s
 * t = perception-reaction time (use 1.4 s)
 * v = speed of approaching vehicles, in ft/s
 * a = deceleration rate in response to the onset of a yellow indication (use 10 ft/s2)
 * g = acceleration due to gravity (use 32.2 ft/s2)
 * G = grade, with uphill positive and downhill negative (percent grade/100)

Red Clearance Interval The function of the red clearance interval is to provide additional time following the yellow change interval in order to clear the intersection before conflicting traffic is released. Red clearance intervals can significantly impact the safety of an intersection by reducing the probability that right-angle crashes occur. The value for the red clearance interval shall be no less than 2 seconds and should be no longer than 6 seconds. A 1-second reduction may be made in the values computed from the red clearance interval formula due to the reaction time delay from conflicting traffic.

$$R=\frac{W+L}{v}$$

Where:
 * R = length of red interval, s
 * W = width of the intersection, in feet, measured from the near-side stop line to the far edge of the conflicting traffic lane along the actual vehicle path
 * L = length of vehicle (use 20 ft)
 * v = speed of approaching vehicles, in ft/s

Gap Time Gap time, often referred to as passage time, is used to extend the green interval based on detector actuations. When vehicles pass over the detectors, this parameter will extend the green interval by a set amount. Once vehicles clear the detection zone the controller counts down to zero from the set gap time value. If no vehicles pass over the detectors during the gap time or the max time is reached, then the phase will terminate and the next phase in the cycle will begin. The below equation can help calculate appropriate gap times.

$$PT=MAH-\frac{L_v+L_d}{v_a}$$

Where:
 * PT = passage time, s
 * MAH = maximum allowable headway, s
 * va = average approach speed, ft/s
 * Lv = length of vehicle (use 20 ft)
 * Ld = length of detection zone, ft

Pedestrian Timing Intervals Pedestrian operations at signalized intersections consists of three intervals:
 * Walk: typically begins at the start of the green interval and is used to allow pedestrians to react to the change to the pedestrian indication and move into the crosswalk.
 * Pedestrian clearance: commonly referred to as flashing don’t walk (FDW). Follows the walk interval and informs pedestrians of when the phase is ending.
 * Solid don’t walk: this interval indicates to the pedestrian that they should have cleared the crosswalk and serves as a warning to pedestrians that opposing vehicle movements are operating. The duration of the solid don’t walk interval is simply the length of the cycle minus the walk and pedestrian clearance intervals.

Walk Interval The walk interval should provide pedestrians adequate time to react to the walk indication and depart the curb before the pedestrian clearance interval begins. The length of the walk interval is typically established by local agency policy. The Manual on Uniform Traffic Control Devices (MUTCD) indicates that the minimum walk duration should be at least 7 seconds. Special consideration may be given for walk durations longer than 7 seconds based on site conditions, such as school zones or areas with large numbers of elderly pedestrians.

Pedestrian Clearance Pedestrian clearance time is computed as the crossing distance divided by the walking speed as shown by the formal below. The MUTCD recommends a walking speed value of 3.5 feet per second (ft/s). A walking speed of less than 3.5 feet per second can be used where pedestrians typically walk slower than 3.5 feet per second, or where pedestrians that use wheelchairs, routinely use the crosswalk.

$$PCT=\frac{D_c}{V_p}$$

Where:
 * PCT = pedestrian clearance time, s
 * Dc = pedestrian crossing distance, ft.
 * Vp = pedestrian walking speed, ft/s

Signal Timing Options

Min Recall

Minimum recall is a very commonly used recall mode. The minimum recall parameter causes the controller to place a call on a certain phase for the duration of the minimum green time. The call is placed regardless of the presence of any detector-actuated calls. A minimum recall is often used where detection has failed. This allows phases with failed detection to continue to function each cycle. This parameter is also frequently enabled for the major street through-movement phases, which ensures that the controller will always return to the major street through phases.

Max Recall

The maximum recall parameter forces the controller to place a continuous call for the max green time on a selected phase. This results in the maximum green duration to occur every cycle regardless to the demand on the phase. This recall mode is similar to a pretimed signal operation, where all phases are set to run for a fixed amount of time each cycle. This recall mode can also be used when detection systems have failed or are not present. Phases in this mode are not allowed to gap out even if gap time parameters have been set. Soft Recall

The soft recall parameter is used to place a constant call for vehicle service on a selected phase when there is no serviceable calls from conflicting phases. When the selected phase has an active green interval, the controller will only serve that phase until the minimum green interval times out. The phase can still be extended if calls are received. The most common use for using a soft recall is during periods of low traffic volumes for the major-road through movement phases (usually phases 2 and 6). The use of soft recall ensures that the major-road through phases will dwell in green when demand for the conflicting phases is absent.

Pedestrian Recall

The pedestrian recall parameter will cause a continuous call for pedestrian service to be placed for a selected phase. This results in the controller using the walk and flashing don’t walk operations. Signals in coordination may often use rest in walk operations, which causes the controller to dwell in the pedestrian walk interval, while awaiting the yield point. Two common applications of pedestrian recall include failed pedestrian detection (bad "push to walk" buttons) and areas with high pedestrian demand. It is a common to use this feature during periods of high pedestrian activity, which typical occur in downtown environments or at intersections near schools.

Coordination Operations The goal of signal coordination is to provide the ability to synchronize signals along a selected corridor for one or more directional movements. Coordinated signal systems are typically found in dense urban areas, on arterial streets where signalized intersections are closely spaced. There are several fundamental parameters that are essential for a coordinated signal system to operate: cycle length, yield point, force-offs, and offsets. These parameters are all necessary for a coordinated system to function. These fundamental parameters are discussed in more detail in the below sections.

Cycle Length

Cycle length is defined as the time required to complete a full sequence of phase movements. In a coordinated system cycle lengths must be the same for all intersections in order to maintain a consistent time based relationship across all intersections. The beginning of the cycle is determined from the specific point in the cycle, which is defined by the user. The beginning of coordination typically occurs with the beginning of the major-street through movement (phases 2 or 6). Cycle lengths can be determined through a variety of different methods and equations.

Yield Point

Yield point can be defined as the deterministic point at which the controller terminates the coordinated phase. This parameter is defined by the user and allows the controller to not end the coordinated phase early. During every cycle, the controller determines what phases will be served next when the yield point is reached. If there are no conflicting calls placed prior to the yield point than the controller will continue to serve the coordinated phase until the next occurrence of the yield point. Figure 4.2.1 shows how a yield point functions within coordinated operations.

Figure 4.2.1: Yield Point and Coordination Example

Force Offs

Force offs are used by controllers to control the phase times by setting points in the cycle to terminate phases even if there is continued demand on phases. There are two modes that are used when programming force offs in the controller: fixed and floating.
 * Fixed Force Off
 * The fixed force off mode maintains a phases force-off point within the cycle. This means that if the preceding non-coordinated phase(s) end early, then the unused time will be allocated to the next phase in the cycle. This can be useful if there are fluctuations in traffic demand, which would allow a phase to receive more green time if other phases do not use their max time. It is important to note that the phase directly after the coordinated phase will never have an opportunity to receive time from a preceding phase, regardless of the method of force-offs.
 * Floating Force Off
 * Floating force off mode will cause the phases to run for their programmed time only. This force off method maintains the non-coordinated maximum times for each of the non-coordinated phases. If a non-coordinated phase does not use all of its allocated time, then the extra time will be assigned to the coordinated phase.

Offsets

An offset is used to define the time relationship between coordinated phases along a corridor of signalized intersections, often expressed in seconds. The offset reference point is used to define a specific point within the cycle where the local controller can measure the relative time difference between the master clock and the coordinated clock. The master clock is a point in time, often referenced at midnight, that is used to establish common reference points between other coordinated intersections. In cases where controllers are connected to a central system (server) through a communications network, the master clock is the central server. Each signal will have their own offset point, which is then referenced to the master clock so that each intersection offset points are relative to each other. This allows the coordinated phases at each intersection to be aligned to allow for synchronized operations between a group of intersections. Figure 4.4.1 represents a graphical example of the relationship between offset reference points.

Figure 4.4.1: Offset Reference Point Example References