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Overview of Traffic Loop Detection Systems
The inductive loop represents the most commonly used method to detect vehicles. The inductive loop is simply a coil of wires embedded into the pavement and can be characterized by several parameters, L (inductance), Q (quality factor), R (active resis- tance, measured using AC signal), and RDC (DC resistance). These parameters are affected by the type of pavement, number of turns in the loop, type of wire, length and type of the
A detector is connected to the loop and is used to measure the AC characteristics of the loop and change of those. A part of the circuitry in the detector and the loop creates an oscillating circuit. The oscillating frequency of this circuit depends on the parameters of the loop and the parameters of the detector. When a vehicle or other metallic mass is located above the loop almost all of the characteristics (except DC resistance RDC) are altered. This change causes the oscillating frequency to drift which in turn is detected by the detector.
Unfortunately, the deterioration of the loop begins virtually from the moment of installation. There appear to be two basic mecha- nisms for loop degradation, mechanical and chemical. Under daily and seasonal temperature variations, the pavement is constantly flexing. To this may be added the pounding of heavy vehicles, particularly at the approaches to signalized intersec- tions where loops are often located. These mechanical factors cause flexure of the wires comprising the loop, especially at the juncture between pavement and berm, leading initially to fine cracks in the insulation, and perhaps ultimately to actual breakage of the wires. After insulation failure occurs (fine cracks or breakage), there is a path for water intrusion. While water itself is a poor conductor, its conductivity is greatly increased by the highly ionic
