6.8 Traffic Detection
An important aspect of traffic-responsive signal control systems is the detection of vehicle and pedestrian traffic demands in order to determine the signal displays required, their initiation and duration. For this purpose, detectors are used to register the presence and/or passage of vehicles and pedestrians.
Detectors can be grouped broadly as vehicle detectors, push-button (pedestrian) detectors and special detectors. Many types of detectors exist including inductive loop, push‑button, microwave/radar, infrared, sonic, video image processing, magnetic and pressure. The most common detectors are inductive loop detectors for vehicles and push-button detectors for pedestrians. Commentary 7 discusses the construction of inductive loop detectors. For additional information on detectors, refer to Klein (2006) and Klein et al. (2006).
For a traffic-actuated system to be effective, it must obtain information on the traffic conditions in the controlled area and the approaches to it and must be capable of providing the following information on a lane-by-lane basis:
- whether there are vehicles and pedestrians waiting against a red signal
- whether filter movements are filtering freely
- whether free-running movements need an extension of green time
- movements of special vehicle types.
These requirements can be met by:
- using a suitable type of detector
- choosing the correct dimensions for the detection zone
- locating the detector correctly in respect to the stop line or where pedestrians will be waiting
- correctly interpreting the data from the detector.
Detectors must have a clearly defined detection zone so that interference from adjacent lanes is low. Currently, the best type of vehicle detector that meets the detection requirements for Australian and New Zealand traffic control systems is the inductive loop detector. Although there are other types of detectors that satisfy these requirements, they are generally operationally or economically inferior in normal situations, and are not considered further in this section.
Abnormal situations or temporary detection requirements during roadworks or situations of road surface instability may be satisfied by use of microwave detector units and video image processing has found increased use in recent years. These detectors are increasingly being used for the detection of pedestrians.