Traffic Detection Technology

Primary Types of Traffic Detectors

Traffic detection systems rely on their ability to sense traffic, which is accomplished through the use of one or more detector types. The two main types of traffic detectors are known as pavement-invasive detectors, which include inductive loop, magnetometer, and magnetic detectors, and non-pavement-invasive detectors based on microwave radar, active or passive infrared, ultrasonic, acoustic, or video image processor technologies.

Pavement-invasive detectors refer to devices that are installed directly onto the pavement surface, into grooves or holes cut into the road surface, under the surface of a roadway by means of tunneling, or anchored directly onto the pavement surface.

Non-pavement-invasive detectors encompass above-ground sensors that are mounted above traffic lanes or on the side of roadways to monitor multiple lanes of traffic at numerous angles. Like subsurface sensors, the above-ground devices are used to measure vehicle count, presence, and passage, but they have the added benefit of also being able to provide information about vehicle speed, vehicle classification, and multilane traffic volume and flow.

Pavement-invasive detectors. The most common traffic detection technology is inductive loop. It consists of one or more turns of insulated loop wire wound in a shallow slot carved into the road surface pavement. Loop detectors come in a multitude of shapes and sizes. Various configurations can be used based on the area under surveillance, the types of vehicles to be detected, and the objective of the data collection, such as counting vehicles or measuring speed. Inductive loop detectors are insensitive to most inclement weather conditions and have a high degree of accuracy, especially regarding vehicle count and occupancy measurement. However, inductive loop installations are subject to temperature and traffic stresses and often require multiple detectors to effectively monitor a specific location.

Another popular pavement-invasive detector type is a magnetometer. These types of detection devices measure changes in both the latitudinal and longitudinal components of the Earth's magnetic field. Magnetometers are especially useful for assessment on bridge decks and viaducts, where the underlying steel support structures tend to interfere with loop detectors and can be weakened by the embedded loop wire. Magnetometers also are appropriate for traffic detection related to temporary installations in construction zones. Like inductive loop installations, however, magnetometers [Page 1359]are susceptible to temperature and traffic stresses and may require multiple installations to effectively monitor a location.

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