Incident Management > Incident Detection > Loop Detectors      Printer-friendly version

What Is It? 

• Inductive loop detectors (ILD) consist of one or more loops of wire embedded in the pavement and connected to a control box, excited by a signal ranging in frequency from 10 KHz to 200 KHz. When a vehicle passes over or rests on the loop, the inductance of the loop is reduced showing the presence of a vehicle. See our Telecommunications Diagram on loop detectors for more information.
• The data supplied by inductive loop detectors are vehicle passage, presence, count, and occupancy.
• For incident detection, loop data is usually relayed to a centralized transportation management center for analysis with a computer based Automatic Incident Detection algorithm.

Key Results

Within the past two decades, loop detector technology has become the most widely used sensor in incident detection systems. They are capable of measuring flow and occupancy, and estimating vehicle speed. They can also be used to actuate traffic control devices and detect congestion and incidents. 

These detectors may suffer from poor reliability mostly due to  improper installation and have high life-cycle costs. In some locations they are not the most appropriate detectors, for example when pavement conditions are unfavorable, on structures or where detection is needed across railway tracks. 

Benefits

• When properly installed and maintained, ILD continues to be the best in all weather, all light condition sensor for many applications.
• It is the most consistently accurate detector in terms of vehicle counts as per the Hughes Aircraft Study.
• The ILD performs well in both high and low volume traffic and in different weather conditions.
• Even with crosstalk problems (at Phoenix freeway site) and a high proportion of lane changes (at the Minnesota signalized intersection site), ILDs had overcounts of only 0.8 percent and 0.4 percent.
• ILDs meet even the most stringent vehicle flow error specifications required by some ITS application.

Disadvantages

• The loop detector system, may suffer from poor reliability, primarily from improper connections made in the pull boxes and in the application of sealants over the sawcut. These problems are accentuated when loops are installed in poor pavement or in areas where utilities frequently dig up the roadbed. 
• Sources of loop malfunction such as stuck sensors can produce erroneous data and may lead to inaccurate detection.
• Another disadvantage of loops is their inability to directly measure speed. If speed is required, then a two-loop speed trap is employed or an algorithm involving loop length, average vehicle length, time over the detector, and number of vehicles counted is used with a single loop detector.

Costs

One of the most significant elements in the choice of detection technology is the life-cycle cost. The elements of the cost that need to be considered include: installation costs; maintenance costs; traffic control; motorist delay and related excess fuel consumption; additional pavement costs; and costs related to increased crash rate during installation and maintenance of some detectors.

Traffic control for a single lane closure can be $1,000 to $1,500 in large urban areas. Loop detectors are relatively inexpensive. Rough component costs are given below (1999 data): 

• Loop with amplifier (purchase and installation) - 700$ per loop 
• Controllers - $2500 per unit 
• Controller Cabinet - $5,000 per unit 
• Fiber optic cable (purchase and installation) - $300,000 per mile

Annual maintenance costs average around 10% of the original installation and capital cost, adjusted for inflation. 

Implementation Challenges

• Unless loops are installed during road construction, installation requires a saw cut, up to 10mm wide into the pavement which may undermine the structural stability of the pavements. 
• Such installation or maintenance procedure invariably delays or disrupts traffic. 
• Loop failures are strongly related to maintenance and installation procedures. Pavement and sealant failure are the main factors that contribute to loop failures. Other reasons include lightning surges, detuned amplifiers, etc.

New Developments

• Recent studies have shown that data from pairs of loop detectors can be used for vehicle reidentification and travel time measurement. 
• Certain new detectors output vehicle inductive signature through a serial port on the detector card. These vehicle inductance patterns can be used to identify vehicles, measure travel times, classify vehicles and determine vehicle origins and destinations.
• Recent studies have also shown that individual vehicle speed estimation can be done using single loop inductive waveforms, output from newer detector cards.

Where is it implemented?

ILDs  are the most commonly used sensor in incident detection systems. They are implemented throughout the world. Most of the European countries report satisfactory equipment performance and reliability. 

Author: Indu Sreedevi