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Speed Measurement

Speeding is always an essential part of evidence before the court in a DUI case. Police departments currently use four primary speed measurement devices: 1) speedometer clocks, 2) radar, 3) average speed computers and 4) LIDAR (Light Detection and Ranging). Two additional types used to a lesser extent are 1) aircraft and 2) photo radar. Each method has its own advantages and disadvantages.

a. Speedometer Clocks

Speedometer clocks have gradually been replaced by more technologically advanced methods, but they are still the least expensive method of clocking speeders and can be extremely effective. The patrol car speedometer is used to pace vehicles. The most important component of this method is an accurate speedometer that is factory certified.

A speedometer can be calibrated several ways: via the fifth wheel attached to the rear of the vehicle; using a stopwatch that has been certified to clock the patrol car over a measured course; or using a dynamometer, which allows the patrol vehicle wheels to rotate in place while the speedometer is checked against the device for discrepancy. While the first two are both effective and inexpensive, the latter, using a discrepancy device, is arguably the best method as well as the most expensive.


An acronym for “Radio Detection And Ranging,” radar involves the transmission of electromagnetic waves that reflect off a moving object. When the wave is reflected, it changes frequency and is interpreted by the radar unit in a speed calculation. This change is referred to as the Doppler effect or Doppler shift. In the simplest terms, the Doppler effect explains how as a sound gets closer to a person, it gets louder. For example, consider the sound a passing car makes as it approaches you then moves away. Radar may be used in both moving and stationary modes.

This is the most popular technology for speed enforcement as evidence by the variety of radar detectors on the consumer market. These devices emit a beeping sound when radar waves are detected, warning drivers of approaching police officers. Despite its popularity, radar use is in litigation across the country due to health concerns regarding cancer risk as a result from frequent use of the radar devices. All recent evidence indicates these claims are groundless, but litigation is still pending. Since most cancer studies involve longitudinal research, 20 or more years may pass before scientists lay this issue to rest.

c. Average Speed Computers

An average speed computer is a device that uses a programmed computer to measure speed by dividing the distance traveled by the time it took to travel the distance. They are typically mounted in police patrol cars and can be used in both a moving and stationary mode.

Whereas radar and LIDAR devices are primarily used to measure maximum speed, average speed computers measure average speed over a specified distance. The advantage these have over radar devices is that they do not use electro-magnetic waves, and thus are undetectable by radar detectors.


On of the more recent devices used in law enforcement for speed measurement is laser or LIDAR (Light Detection And Ranging). LIDAR devices use an infrared light wave emitted at frequencies that allow the beam to be focused into an extremely narrow target area. The devices are usually operated in the hand-held mode. Although they can be used through the glass it reduces the device’s range; therefore, an open window or exterior use is preferred.

LIDAR has become more popular with the frequency of consumer’s radar detectors. Detection of laser beams is possible but the devices that detect laser beams are limited in their effectiveness. This is due to the fact that when the device intercepts the laser beam, this corresponds to the clocking of the vehicle with the LIDAR device.

In addition, most LIDAR devices are mounted inside the vehicle, further limiting their detection by another device. The theory behind laser technology is that speed is calculated by dividing the distance by the time of the light pulses of the laser (S=D/T of light pulses).

e. Aircraft

This method of speed enforcement uses the combination of ground-based units and a fixed wing airplane. This method of enforcement is based on the formula Speed = Distance/Time.

Law enforcement uses painted lines on the pavement to identify a measured course. Then as vehicles travel on the measured course, a stopwatch is activated in the airplane. Once the course is completed, the speed is calculated and, if the vehicle was speeding, the description is broadcast to the ground units. The vehicle is pulled over and the vehicle and speed are verified.

It is for this reason truckers will commonly warn fellow truckers via radio of hovering law enforcement. The aircraft, typically the high-wing design that allows an unobstructed view of the ground, can also be used for marijuana eradication activities, emergency transport, traffic monitoring, surveillance and other law enforcement programs.

f. Photo Radar

An extension of traditional radar devices, this technology uses photography to capture the vehicle and license plate when the violation occurs. The date, time and speed can be superimposed onto the photograph. Some devices are so accurate that they can also capture the driver’s image in the picture.

Photo radar can be used in manned or unmanned applications such as those devices installed in lights at busy intersections. It is commonly used in jurisdictions where specific legislation permits its use and where vehicles have both front and rear plates.

g. Drone Radar

Drone radar is essentially an unmanned radar station that purposefully triggers motorists' radar detectors. When the detector alarms sound, it is presumed that drivers will slow their vehicles wary of a police officer that is not actually there.

These units can be mounted in moving vehicles, concealed in highway signs, or installed in highway work vehicles and any variety of other locations. The FCC and NHTSA have regulations that must be met in order to use this method of speed enforcement. As motorists catch on, overuse of this method will reduce its effectiveness.

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