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Other Roadside Detectors

What is it?

• Roadside detectors are sensors that cause minimal disruption to normal traffic operations and do not need to be installed in or on the pavement. They are generally mounted on overhead structures or to the side of the pavement.
• Roadside detection technologies include passive/active infrared, microwave, radar, ultras onic, and passive acoustic detectors. These can be used to count traffic volumes, measure vehicle speeds, and/or classify vehicles. See our Telecommunications Diagram on Roadside Sensors for more information.
• There are typically two types of roadside detectors: passive (i.e. they detect energy that is emitted by the vehicles and roadway or is reflected from them) and active (i.e. they transmit energy, a portion of which is reflected or scattered from the vehicle and roadway back toward the sensor).

Key Results

Most of these results are taken from the Minnesota Guidestar Program Final Report

• In general, the differences in performance from one roadside detector to another within the same technology are more significant than the differences from one technology to another.
• Vehicle Speed
  • Radar, Doppler microwave, and video are the most accurate technologies for measuring vehicle speeds.
• Traffic Detection
  • Passive and active infrared technologies show potential for detecting traffic at intersections and/or freeways.
  • Doppler microwave and radar technologies have good potential for detecting traffic and measuring the speed of moving vehicles.
  • Pulse ultrasonic technologies have good potential for detecting traffic at both intersections and freeways.
  • Passive acoustic technologies have a moderate potential for detecting traffic at intersections and freeways.
  • Video image detection requires extensive installation and setup time and is not as accurate as other technologies, but it has the advantage of offering a wide variety of traffic data.
• Vehicle Counting
  • Most of these technologies are well suited for temporary counting situations.

• Roadside technologies are relatively easy to install and do not require any intrusive reconstruction of the pavement or roadway.
• Because they are non-intrusive, the installation of these technologies does not interfere with traffic flow.
• Non-intrusive detectors provide fairly accurate measurements of vehicle speeds, traffic volumes, and vehicle counts.

CostsThe following tables are extracted from the report: Evaluation of Some Existing Technologies for Vehicle Detection from the Texas Transportation Institute by Middleton, Dan, Jasek, Debbie and Parker, Ricky, September 1999. This extensive report contains a more detailed performance review and discussion for each of the aforementioned detectors. Link to Specific Vendor Information  

Quantitative Evaluation and Cost Estimates at Signalized Intersections

Quantitative Evaluation and Cost Estimates on Freeways

Performance Evaluation of Detectors at Signalized Intersections

Implementation and Operational Challenges

• Extremely cold weather makes access to roadside devices difficult and can even cause some cables to split open.
• Some weather conditions like snow can cause vehicles to drive outside of their usual driving behavior, which may affect detectors with narrow detection zones.
• Lighting conditions can affect video devices, particularly in the transition from day to night.
• There is an on-going challenge to develop devices that incorporate multiple technologies (i.e. passive millimeter microwave and infrared video).

Where is it Implemented?

Non-intrusive, roadside detectors are widely used throughout the United States, Europe, Japan and Australia and on a more limited basis in Asia and Latin America.