Vehicle Detection

Vehicle detectors and sensors commonly used for traffic monitoring include:

• Surface mounted or below ground:
  • inductive loops
  • magnetic field sensors (also above ground)
  • galvanic contacts / treadles
  • piezo and capacitive strip sensors (typically used for axle counting)
  • bending plate scales (typically used for weigh-in-motion)
• Above-ground:
  • Closed Circuit Television (CCTV) cameras
  • microwave (radar) detectors
  • laser radar detectors
  • magnetic field sensors
  • passive or active infra-red sensors
  • ultrasonic and other instruments based on acoustic or video image processing

CCTV Cameras

CCTV cameras are often used as a substitute for inductive loops in some applications and when interpreted by computer can provide valuable information to road operators about traffic flows, incidents, local weather conditions and the presence of pedestrians and animals. Images can also be used to derive information on speed, occupancy and traffic volume. It is also one of the main sensor technologies for Automatic Incident Detection (AID). (See Automatic Incident Detection) A typical CCTV-based detection scheme consists of cameras installed on masts, each having a roadside cabinet, located on the edge or the median of the road. The topography of the road network dictates where cameras should be located (and their height) - to ensure some overlap of coverage zones. Camera separation for AID is likely to be denser than for general surveillance.

Applications

Point sensors and cameras are used for incident detection and warning as well as for the daily management of the network:

• inductive loops are the most common type of point sensor and are usually owned and operated by a traffic authority
• a dual loop configuration (two sets of loops, typically separated by 2 or 3 metres) can provide a spot speed measurement and – when installed a few hundred metres apart on main roads or intersections – can be used to measure flow rates and to manage traffic by imposing variable speed limits
• galvanic contacts and treadles are often used in toll lanes to count the number of vehicle axles – by counting the number of times that the contacts close in response to wheel pressure
• piezo or capacitive strip sensors can detect presence and instantaneous weight and are often used for dynamic Weigh-In Motion (WIM) schemes
• CCTV cameras are the most common device for traffic monitoring since the images and the processing of the images provide information that can be easily interpreted by a road operating authority for network control, surveillance and incident detection
• other above ground detectors are generally focused on a small area such as a single lane, a strip across a road or an intersection - to provide some combination of presence, speed, occupancy and vehicle classification data (See ITS Technologies)

There are advantages and disadvantages to using the different types of detectors:

• surface mounted detectors: require a traffic lane to be closed when a sensor is replaced – so care must be taken to install these detectors on a stable sub-surface that makes them less prone to mechanical movements that could cause them to fail
• above ground sensors: can provide a view across a larger detection zone, and are easier to replace when faulty - but are also more likely to be vandalised unless properly protected. Microwave (radar) detectors have the advantage that they can be mounted on poles beside the road - but they may not always be as accurate as inductive loop detectors and can be impacted adversely by rain, particularly over extended distances

In the urban environment, buried inductive loops are inexpensive but require on-going maintenance to ensure that a significant proportion remain operational. Above ground detectors may be more appropriate instead.

For remote locations, the cost of providing power and communications for detection equipment and CCTV cameras can be high, particularly if the security of the camera sites and communications cables cannot be ensured. In these circumstances, solar-powered sensors or cameras equipped with cellular radio modules can be used. Limited light (particularly in winter months) and variable radio coverage may mean that these installations are equally unreliable.

Cameras with overlapping coverage can increase the resilience of the camera network. When combined with Automatic Incident Detection (AID) and other sensors, a comprehensive view of the traffic network can be progressively developed. This can be expensive. (See Vehicle Detection and Probe Vehicle Measurement)

The data gathering process can capture information that, when used by itself or when combined with other data, would be regarded as ‘personal information’ in some countries’ legal frameworks, raising issues of data protection and privacy. (See Legal and Regulatory Issues)