A traffic surveillance system is commonly characterized by its use of traffic sensors to collect necessary data, a transmission network to transmit the data to a central data bank, and a digital computer system with peripheral devices to analyze the data bank, and a digital computer system with peripheral devices to analyze the data and provide information for the management decision process. Arterial streets are the major traffic facilities of concern, and traffic surveillance and control on arterial streets reduce congestion. Placing detectors at strategic locations along the arterial streets assists in the decisions to balance demand with capacity at congested locations. The two basic measurement variables, volume and occupancy, are used for traffic control and management of the arterial street system. Volume can be easily and accurately obtained by counting the number of pulses measured during a given time period.
Occupancy is the percentage of time that a detector is indicating the presence of a vehicle.
Research as indicated that an occupancy level of more than 25 percent is an indication of the onset of congestion. The objective of the detector system is to provide input for the adjustment of the signal timing to reflect the upstream changes in major traffic flow. System detectors are strategically placed at mid-block locations in an effort to increase the efficiency of progression along arterial streets. The control parameter can be computed by a master supervisory station by using the inbound and outbound traffic flow data. Detection on cross-street approaches and high volume driveways are similar to those for normally operated local intersection control.