Vehicle Speed Measurement Methods

Throughout history, law enforcement officials have utilized a wide array of speed measurement devices which have varied greatly in their ease of use and accuracy. As of now, police departments use four primary devices to measure speed:

  • Speedometer Clocks
  • Radar
  • Average Speed Computers
  • LIDAR (Light Detection and Ranging)
  • Aircraft
  • Photo Radar
  • Drone Radar

Aircrafts and photo radar are sometimes used as well, but to a lesser extent. There are pros and cons to each method.

Speedometer clocks are the least technologically advanced method of measuring speed and have been largely replaced by more efficient devices. However, they are still used in some places because they are the least expensive method of clocking speeders and report fairly accurately.

Radar is an acronym for “”Radio Detection and Ranging.”” This method of speed clocking measures the transmission of electromagnetic waves as they reflect off of a moving object. When a wave is reflected, its frequency changes, and the radar interprets the discrepancy in a speed calculation. This change in frequency is commonly known as the Doppler Effect or Doppler Shift.

Radar is the most popular method for enforcing speed limits. Because of this, many consumers choose to purchase radar detectors, which are able to determine when a speed-detecting radar is nearby. The device is used with the intention of reducing the driver’s risk of being pulled over.

An average speed computer is a technological device that uses a computer program to measure speed. It does this by dividing the distance traveled by the time it took to travel that distance, which provides the average driving speed over the course of that distance. These devices are often found in police patrol cars. Because they do not use electro-magnetic waves, they cannot be detected by radar detectors.

LIDAR, or laser, devices use an infrared light wave emitted at frequencies that allow the beam to be focused into a narrow target area. These are usually hand-held devices used outside of the patrol car because the glass of the windshield can reduce the range of the device. The theory behind laser speed detection technology is that speed is calculated by dividing the distance by the time of the light pulses of the laser (S=D/T).

While it is possible to detect the laser beams emitted by LIDAR, the devices that do so are limited in their effectiveness, which makes this device the first choice of many law enforcers.

Aircraft are sometimes used as speed measurement tools. In this case, speed enforcement is achieved through a combination of ground-based units and a fixed-wing plane. The formula for this method is Speed=Distance/Time.

To use this method, lines must be painted on the pavement to identify a course. As vehicles travel on the designated course, a clock is activated on the airplane. After the course is completed, the speed is calculated, and if the vehicle was driving over the speed limit, the ground units are notified.

A photo radar device is like a typical radar device except that it takes a photograph of the vehicle and license plate of the speed offender. The date, time, and speed can then be added to the photo file. Some devices are so advanced that the driver’s image can be captured as well.