Traffic

From A TRAFFIC DETECTION TOOL KIT FOR TRAVELER INFORMATION SYSTEMS from Mark D. Suennen at the Maryland Department of Transportation State Highway Administration

Loop detectors are considered a mature device; highway agencies have been using loop detectorssuccessfully for over 20 years. Much is known about their operation, capabilities, advantages anddisadvantages. An inductive loop detector is composed of one or more turns of a loop wire installedwithin the pavement, connected to a lead-in cable, which in turn is connected to an electronic receiver. Acurrent is passed through the loop wire, creating an electromagnetic field about the wire. A metallic bodysuch as a vehicle passes over the detector, producing a change to the electromagnetic field of the loop.The change is registered as a vehicle passage or presence (1).

Individual ILDs are capable of measuring the passage of a vehicle over a particular point, the volume, andtime occupancy. If one adds a second loop detector a known distance from the original (known as a speedtrap), one can measure the vehicle’s speed, as well as detect changes in vehicle behavior (i.e. slowing foran incident, increasing speeds), some classification of the type of vehicle and wrong way movementdetection (1). A single loop detector is typically 95 percent accurate but can vary by as much as 20percent with failure rates between 5 percent and 15 percent if not properly installed and maintained (2,3). Loop detectors have been and continue to be one of the least cost alternatives available with a 10-year life cycle cost of nearly $7,700 (2).

The disadvantages of ILDs have been well documented over the decades of use. Improper installationcan cause large errors in readings or no readings at all. As ILDs are in-pavement sensors, the roadwaymust be blocked during installation and maintenance activities, causing motorist delay and risking thesafety of the workmen. If a pavement is in poor condition, the loop detector cannot be installed withoutfirst repairing the roadway. Loop detectors cannot be effectively installed in reinforced concrete such as abridge deck nor in pavements where the iron content of the surrounding soil is high. Buried high voltagelines can sometimes lead to mistaken signals. Trucks with a higher than average body can sometimes bemisinterpreted as two consecutive small vehicles, closely spaced. Motorcycles can be missed fordetection if the sensitivity of the detector is too low. Relocation of the loop detector is difficult andtypically more costly than replacement. Crosstalk, the interaction of two individual loop detectors, canlead to double counting. The minimum distance recommended for pairs of loop detectors is 24 inches forlow sensitivity, 36 inches for medium sensitivity, and 48 inches for high sensitivity (4).

1. P. Michalopoulos and C. Anderson, “Costs and Benefits of Vision-Based, Wide-Area Detection in Freeway Applications”, Transportation Research Record #1494, National Academy Press, Washington, D.C., 1995.

2. D. Hartman, D. Middleton & D. Morris, “Assessing Vehicle Detection Utilizing Video Image Processing Technology”, Report No. FHWA/TX-97/1467-4, Texas Transportation Institute, College Station, TX, 1996.

3. Conversation with Dan Middleton, Texas A&M University System, June 2000.

4. V. Bhagat and D. L. Woods, “Induction Loop Detector Systems Crosstalk”, FHWA/TX-94/1392-2, Texas Transportation Institute, College Station, Texas, 1992.