Inductive Loops

Inductive loops are the electronic bedrock of the traffic management system, with more than fifty thousand of them in the metropolitan area. Loops are generally composed of an insulated wire, set into a shallow trough cut into the pavement to form a square or circular “loop” approximately six feet wide. Each loop is usually visibly linked to a connection point at the curb, and then wired to the traffic control system. The magnetic field generated by low voltage running through the loop is altered by large metal objects passing over it, and this disturbance is detected by the loop and registered by the electronics connected to it on the curb. Each vehicle passing over a loop is a click in the system. Complex intersections can have dozens of loops, and major streets and highways are dotted with them. A series of loops can measure the speed at which cars travel, as well as how many are on the road. Intersections can count the cars that line up before automatically triggering the signal.