Traffic detectors

Traffic detectors are a critical part of any intelligent transportation system. They collect data about the current traffic conditions so that transportation management and operations decisions can be made.

Key characteristics



WSDOT regions

Strategy description

Traffic detectors supply the data that is needed to operate most intelligent transportation systems.

Some functions of traffic detectors include:

  • Counting the number of cars, trucks, pedestrians, and bicyclists
  • Detecting transit or emergency vehicles
  • Measuring vehicle speeds, how long it takes for vehicles to get from one location to another, and congestion levels
  • Detecting parking availability
  • Informing traffic signal operations
  • Collecting weather data
  • Monitoring near collision misses 

For example, a radar detector on a highway counts cars, trucks, and busses, as well as  measures speeds. An agency may use that radar detector data to inform a variable speed system, a ramp metering system, and calculate travel times to post on an electronic message sign.

There are two basic types of traffic detectors to collect this data:

In-roadway detectors, such as inductive loops, magnetometers, and roadway temperature sensors that are embedded in the pavement.


  • Inductive loops have been used for decades and reliably detect and count vehicles


  • Once installed, in-roadway detectors are permanent and require physical construction to change their location
  • They can fail when pavement deteriorates
  • They must be replaced when roadway construction occurs

Over-roadway detectors, such as video cameras, radar, Bluetooth/wifi, and weather sensors, that hang on a pole. These technologies have improved significantly over the past decade and may be an effective alternative to in-roadway types.


  • They continue to operate during construction, and the operator can change the detector layout using the software
  • No physical changes are needed to relocate the detectors


  • Weather conditions, such as fog, can negatively impact some over-roadway detectors, like video cameras.

When to use this strategy

Traffic detectors make sense for any intelligent transportation system that needs data about traffic conditions (volumes, speeds, occupancy, travel times), users of the system (pedestrians, bicyclists, transit) or roadway conditions (temperature, congestion levels).

When selecting the traffic detector technology, consider what systems will use the data which can impact costs and equipment needs. A low-cost example is a driver speed feedback sign that has only a single speed detector and an electronic sign. In contrast, a signalized intersection can require detectors that cost over $100K for a single intersection. In addition, multiple detectors are typically required to complete a system, which increases the cost.

Strategy benefits include:

  • Providing the critical data needed to inform most transportation management systems and monitor the effectiveness of the transportation strategy.
  • Enabling agencies to automate traffic control systems, such as traffic signal control, variable speeds, ramp metering, and more, so the systems adjust to the current traffic conditions.

What you need in order to implement

Planning needs:

  • Determine how to plan for and choose the best types of traffic detectors based on data needed to manage and operate the transportation system
  • Select the detector technology and plan the detector location based on the needs of the system.

Equipment needs:

One or both basic types of detectors:

  • In-roadway detectors, such as inductive loops or magnetometers
  • Over-roadway detectors, including video cameras, radar, Bluetooth/wifi readers, and weather detectors

Communications and power cables from the detector to the field computer/controller

Maintenance needs:

  • Repair or replacement of traffic detectors when they fail. Many systems also monitor the health of the detectors and alert maintenance personnel when a detector has a problem.

Learn more about this strategy

Federal Highway Administration (FHWA), Traffic Detector Handbook (PDF).

About key characteristics

Location notes:

Traffic detectors are a supporting technology that can be used in any location to collect data about current transportation conditions.

Cost notes:

The cost of installing traffic detectors can vary widely depending on system needs.

Technology notes:

The technology needs for traffic detectors are medium because traffic detectors must be combined with other technology to provide data to the transportation system. There are multiple detector options available and the designer must understand the detection capabilities.

Collaboration notes:

Traffic detectors can be installed as part of a stand-alone, single-agency transportation system. Collaboration may be necessary when a system crosses agency boundaries, but typically an agency selects the detection technology they prefer without collaborating with others.

Need or issue this strategy addresses