How Traffic-Actuated Signals Work

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. When the detector registers a vehicle and transfer information 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 the 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.

Induction loop detectors are popular detectors despite their difficult installation. Video imaging detectors are increasingly used because they do not require installation in the ground and a single camera can provide detection for an entire approach. loop detectors are also popular for they can be installed on the road surface. Here introducing two main settings up ways for detectors. 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. Another option that is available when using detectors is a delay.

How Traffic-Actuated Signals Work

Example of a loop detector

How Traffic-Actuated Signals Work

Example of a magnetometer (wireless sensor)

Detector Placement

For controllers to communicate with the detectors, they must be properly numbered. Most detector numbering schemes are based on phase numbering and depend on the type of controller used.

Other Vehicle Detection System

The new trend of sensors are magnetometers which are flushed into the road and send a signal wirelessly to a router installed beside the junction (which directly communicates with a traffic controller) to provide vehicle detection for various applications such as fully-actuated signaling or running a red light enforcement and the like.

How Traffic-Actuated Signals Work

Example of wireless sensors flushed into the road

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 pre-timed 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.

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.

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 top bar. This is why there are often multiple detectors used at intersections.

Pedestrian Movements

In the discussion of pedestrian signals, it was typically assumed that the green times would be long enough to allow pedestrians enough time to cross the street. Pedestrian signals are not too different from the normal vehicle signals; there are 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.

How Traffic-Actuated Signals Work

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