How do the traffic lights know when to turn?
"In general, there are two types of traffic signal control. One is the so-called fixed-time signal, and the other is the vehicle-actuated signal.
"Fixed-time traffic lights operate on timers. The traffic lights turn green according to a pre-determined sequence, and green-light duration serving each direction is fixed, no matter how the traffic condition changes. Fixed-time traffic signal may have multiple timing plans, which correspond to different times of day, such as morning peak and afternoon peak.
"Vehicle-actuated traffic signals, on the other hand, are smarter than fixed-time signals because they can adapt to traffic conditions. Detectors are installed at intersection and vehicle-actuation of the detector will register a call at a signal controller. The registered call indicates a request for green-time allocation and the request will be served as soon as previous requests have been served.
"If the request comes from an approach already being served, the green time may be extended. Therefore, when more cars are detected in one approach, the green allocation for that approach is longer unless the maximum green time is reached. The green light for one approach can be also terminated if the gap-time between two vehicle-actuations is longer than the preset threshold, indicating the presence of non-continuous traffic flow.
"There are all sorts of technologies for detecting cars, and by far the most common technique is the inductive loop detector. An inductive loop is simply a coil of wire embedded in the road's surface. You can often see these big rectangular loops cut in the pavement because the compound is obvious. The traffic signal system constantly tests the inductance of the loop in the road, and when the inductance rises to certain threshold, a vehicle is detected and traffic signal system will allocate green time accordingly."
Henry Liu is an assistant professor in the Department of Civil Engineering.
