An inductive loop is a metal detector installed in the surface of the roadway.
Permanent Inductive Loops are considered the most reliable traffic detection method available. An inductive loop consists of wire "coiled" to form a loop that usually is a square, circle or rectangle that is installed into or under the surface of the roadway.

Inductive loops are used in signal detection and work like a metal detector. Once a vehicle drives over a loop sensor the loop field changes which allows the detection device to detect the presence of an object (mainly a vehicle).

Often Inductive loops are referred to as presence detectors, and in traffic detection are often used in combination with axle sensors to collect classification data.