Traffic Signal Operations

Main Page>Technical Documentation>Traffic Signal Operations Signal Operation Modes

There are four main modes of operation for traffic Signals:
 * Pretimed
 * Semi-Actuated (Uncoordinated)
 * Actuated (Uncoordinated)
 * Fully Actuated (Coordinated)

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 (uncoordinated) 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 is not coordinated because cycle lengths can very between cycles.

Actuated (uncoordinated) signals have vehicle detectors for all active phases. Phase timing will vary based on vehicle actuations. Signals running in this mode are typically isolated and are not near any other signalized intersections.

Actuated (coordinated) signals behave in much the same way as actuated uncoordinated signals. Actuated coordinated signals typically are near other signalized intersections and therefore require coordination between other signals. It is also common for signals in this mode to have unused time from other phases allocated to the coordinated phase.

SwashSim currently uses two traffic signal modes: pretimed and actuated (uncoordinated). Each mode is explained in further detail within the Network Setup section. Basic Signal Timings

The minimum green parameter represents the least 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.
 * Min Green

The maximum green 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.
 * Max Green
 * Yellow Change Interval

For Approach Grades other than 0% use ITE formula:

$$Y=t+\frac{1.47v}{2(a+Gg)}$$

Where:
 * Y = length of yellow interval, sec.
 * t = perception-reaction time (use 1.4 sec.)
 * v = speed of approaching vehicles, in mph
 * a = deceleration rate in response to the onset of a yellow indication (use 10 ft/sec2)
 * g = acceleration due to gravity (use 32.2 ft/sec2)
 * G = grade, with uphill positive and downhill negative (percent grade/100)


 * Red Clearance Interval 

$$R=\frac{W+L}{1.47v}$$

Where:
 * R = length of red interval, sec.
 * 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 mph


 * Gap Time

Signal Timing Options

Min/Max/Soft Recall

References