Radar, which stands for Radio Detection and Ranging, works by transmitting electromagnetic waves that bounce off of a moving object. After the wave is reflected by an object, it changes frequency, and the radar machine interprets the frequency in a calculation of speed. The change is known as the Doppler Effect or Doppler Shift.
Radar speed detectors are the most popularly used method for identifying speeding motorists. The radar signal given off by the detector spans across all lanes of traffic and in both directions. When used for objects less than one-quarter of a mile away, the beam can expand to be as wide as six or eight lanes of traffic.
The detector reports information on the strongest signal it receives, and the size and shape of a vehicle can affect the strength of the signal. For example, large trucks reflect stronger signals than motorcycles or small cars. Consequently, the radar speed detectors can sometimes report on vehicles that the police officer did not intend to check.
This method is usually quite effective in terms of locating offenders. When in good condition and used properly, radar detectors have only a 1/10-mile-per-hour margin of error.
Speed detectors that use radar can work in two different ways. The first, called the “”pulse mode,”” measures the time between the transmitted wave leaving the radar unit and wave reflected from the target. Once the speed of the radar waves is realized, the time it takes the wave to reach the target is measured, and its distance is calculated. Measuring the distance over a number of pulses allows for the calculation of the speed of the vehicle.
With the second method referred to as “Doppler mode,” radio waves reflect off of a vehicle with a small change in their frequency. The amount of change is used to determine the speed of the vehicle.
In both of these methods, the police officer points the radar gun at an approaching car, and the device automatically calculates the vehicle’s speed and displays it on a small screen at the back of the radar gun. If the motorist is speeding, the officer can flag down the car or alert other members of the force further down the road to pull over the car.