Actuated Signals Background
There are many aspects to actuated signals and many of them are covered here. You can click on a link to be taken directly to a section, or just start at the beginning and proceed to the end.
Operations
Actuated signals work like this: Some sort of detector at the intersection registers whether or not there is a car at the intersection. Fully-actuated signals have detectors on all of the approaches and semi-actuated signals only have detectors at some of the approaches. If the detector registers a vehicle it makes a call to the controller and the controller registers the need for the phase to be serviced, or have the right of way. When this phase is in service, it retains the right of way for a minimum time and additional time can be given if more cars are detected during the green light. This additional time is called the passage time. If there is enough traffic, extensions (in the form of passage time) will be added to the phase up to some set maximum green time. However, if another call is not received during the green time, the phase will end. This is known as gap out. The cycle then progresses based on calls received during the previous cycle. The signal controller then changes to the next phase in the phase sequence that has a call.
Detectors
There are several different types of detectors currently in use but the most common are induction loop detectors. Video imaging detectors are increasingly being used because they do not require installation in the ground and a single camera can provide detection for an entire approach.
Loop detector on the ground. These detectors are intended to be installed in the road surface.
Detectors can also be set up in a few different ways. First of all, they can be set to pulse detection or presence detection. Pulse detectors simply send a quick pulse to the controller, no matter whether the vehicle stays within the range of the detector or it just passes through. Presence detectors place a call for the entire time that a vehicle is detected in the range of the detector. With presence detectors, you have the option of letting the controller remember a call or not. This setting is known as memory and is useful for left and right turn movements. If memory is turned off and a vehicle arrives and is serviced (leaves) before the signals change, then the call will be “forgotten” and the phase will not be served. Another option that is available when using detectors is delay. This function will delay the placement of a call until the detector has been occupied for a preset amount of time. This setting is useful for permitted right and left turns by giving the vehicles a chance to proceed before the movement is given the right of way.
Detector Placement
For controllers to communicate with the detectors, they must be properly numbered. The numbering schemes are generally set by the specific agencies, so we will not go into much detail here. Most detector numbering schemes are based on the phase numbering and depend on the type of controller used. In the NIATT laboratory, our controllers have the capability to accept 24 different detectors, but are set up for 8 on the simulations. The easiest way to keep track of the detectors is to number them according to which phase they are monitoring. For example, if you have a detector on the northbound left turn lane, it should be detector number 1. Even if that movement does not have its own phase, this numbering scheme is convenient, and the phase can still be assigned to the proper movement in the controller.
Maximum Green Time
For actuated signals, it is common practice to set the maximum green time by determining the green splits for the intersection as if it were pretimed and then multiplying the values by 1.25 to 1.5 (Roess et al). This is just a guideline; as with most aspects of actuated timing there is a great degree of judgment involved. For example, if the location is subjected to large variations in flow (near a popular lunch stop, a school or church, etc.), then the maximum green time may need to be set higher to accommodate these abnormal flows.
While it might be intuitive to assume that there is no need for a maximum green at actuated signals, that the detectors will ensure a smooth operation, this is not necessarily the case. The maximum green time is set so that if there are calls on other phases they can be serviced without waiting for the first phase to gap out. If we set the maximum green times too high, sustained high demands (such as at rush hours) would start to affect operations on other approaches. The maximum green setting helps to contain phase failures (intervals during which queues do not clear) to a single phase.
Minimum Green Time
Minimum green time is set to the minimum amount of time required to clear the vehicles potentially stored between the stop bar and the detector (advanced detection). Vehicles not in this area when the phase goes active will trigger a vehicle extension, so this minimum ensures that the signal does not forget about the vehicles downstream of the detector. The minimum green time can be calculated using the following equation:
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For the equation above the following values are generally used: a startup time (ts) of 4 seconds, a headway (h) of 2 seconds, and a vehicle length (Lv) of 20 feet.
Theoretically, if the detector were placed at the stop bar, the minimum green time could be set to six seconds. However, it is often desirable to set the minimum green time slightly higher, to as much as 10 seconds, to ensure that there is ample time for the queue to be serviced without a premature gap out.
Recalls - Maximum, Extendible, and Soft
Passage time is generally taken as the time required for a vehicle to travel from the detector to the stop bar (advanced detection) and is also the maximum gap time. For stop bar detection it takes no time to travel from the detector to the stop bar, so the passage time will go to zero. There can clearly not be a gap of zero seconds between successive cars. For this reason, if the passage time is too small it is arbitrarily set to three or four seconds. This is more than the assumed headway of two seconds, but not excessively large to cause the signal to max out every phase. If there is advanced detection, it is often the case that the passage time has to be excessively large to allow a vehicle enough time to get from the detector to the stop bar. This is why there are often multiple detectors used at intersections.
Here is a demonstration on how passage time works.
Pedestrian Movements
When setting up actuated signals, it is important to remember the pedestrians. In the discussion of pretimed signals it was typically assumed that the green times would be long enough to allow pedestrians enough time to cross the street. In actuated operations, the minimum green time given to a phase can be as low as 5 seconds, which will generally not be enough time for a person to cross the street. For this reason, pedestrian buttons and signals are installed. Pedestrian signals are not too different from the normal vehicle signals; there is a walk interval and clearance time. The walk interval is indicated by a WALK indication and it is during this time that pedestrians are supposed to start walking across the street. Generally, the minimum walk time given to pedestrians is between 4 and 7 seconds, though it can be longer. The clearance time is indicated by the flashing DON'T WALK indication and provides enough time for a pedestrian walking at 4 ft/sec to get from the sidewalk to the edge of the last lane of traffic. The DON'T WALK signal indicates that no pedestrians should be in the crosswalk. Depending on the agency, the DON'T WALK interval can start during the coinciding traffic green or yellow and will almost always be given during the all-red interval.
Here is a demonstration on how pedestrian movements interact with vehicular movements.
Continue on to Actuated Exploration.