Using Dual Loop Speed Traps To Identify Detector Errors

Dual loop speed traps have a distinct advantage over single loop detectors because the speed trap detection system is redundant. Each vehicle is observed twice under normal operating conditions, once at each loop. The two observations are normally used to measure velocity, but as this paper demonstrates, the redundancy can also be used to assess the performance of the speed trap and identify detector errors. At free-flow velocities, the time each detector is occupied by a vehicle (i.e., the ontime) should be virtually identical, regardless of the vehicle length. Many hardware errors will cause the two on-times to differ. Exploiting this property, a formal methodology for testing speed traps off-line has been developed, and ways to extend the work to on-line testing are suggested. The work is used to evaluate several loop sensor units, revealing problems in two models. A second example shows how the work can be used to detect cross talk between sensor units.