Commentary 9 Advance Loop Detectors
Advance detector loops may be used in addition to the normal stop line detectors. They can be set in presence or passage mode. In the passage mode of operation an inductive loop detects the passage of a vehicle passing over the loop, however, it does not detect the duration the vehicle is over the loop as in the presence mode. Advance detector loops may also be used without stop line detectors as shown in Figure C9 1.
Generally, advance detector loops are located at a distance from the stop line that corresponds to the actuated signal gap setting. Under light to moderate traffic conditions the use of advance detectors can lead to a reduction in delays and stops under light to moderate traffic flows by tending to:
- provide an advance call for a relevant phase
- avoid the termination of a green display when a vehicle is in the ‘dilemma zone’.
Advance loops are able to terminate phases earlier, since assessment of gaps can be made several seconds before it can be detected at the stop line.
Several considerations also apply to advance detector loops. In some control strategies, advance detectors are used in a gap-seeking role on high-speed approaches or where there is a large number of heavy vehicles to enable the onset of a gap to be identified earlier. However, because of the long distance from the stop line, they have the following shortcomings:
- They are not as effective as stop line detectors in identifying turning movements if placed upstream of exclusive turning lanes.
- Demands lodged are processed on the assumption that vehicles do not change lanes or turn off before reaching the stop-line.
- Vehicles entering the roadway between the detector loop and the stop line are not detected, or vehicles leaving the roadway between the detector loop and the stop line are detected unnecessarily.
- They cannot detect slow-moving vehicles, queues or stationary vehicles if operating in passage mode.
- Arbitrary provisions have to be made for the green time requirements for traffic trapped between the detector loop and the stop line at the start and end of the phase.
- Excessive allowances need to be made for the time necessary for individual vehicles to travel from the loop to the stop line during the green interval.
- The further the detector is located away from the controlled area (i.e. the stop line), the less accurately it is able to respond to changes in traffic flowing into the controlled area (e.g. a decrease in capacity and queue formation).
- The installation of detector loops far in advance of the stop line is generally unattractive from an economic point of view.