How Radar Measures Speed

Posted Friday, October 26, 2018 by Andrew Charles Huff

If you’ve ever been pulled over for speeding, you know that your vehicle speed was probably measured by either a laser or radar Speed Measuring Device (SMD). Most police agencies in the state use both types of technologies to determine speed.

How “Radar” Really Works

RADAR stands for Radio Detection And Ranging. It is a general term for the process of determining the range, angle, or velocity of objects. Modern traffic radar uses the Doppler effect, which is an increase or decrease in the frequency of waves traveling between an observer and an object.

An example of the Doppler effect in everyday life is when you hear a high-pitched ambulance siren approaching, and then it gets lower-pitched when driving away from you. The Doppler effect doesn’t just apply to sound. Light also travels using wavelengths and this is how officers measure speed.

How do Police Use Radar for Speed Testing?

When measuring for speed, an officer will typically use an SMD to direct a beam of light toward an object. The SMD measures the time it takes for the beam to be reflected back to the device. This split-second measurement is incredibly precise, and estimates the object’s speed.

If an officer is stationary sitting in their patrol car, they will aim the SMD at the vehicle to measure the speed. If the officer is moving while using the SMD, the device will measure the difference in speed between the moving police or patrol car and the suspect vehicle. The device will then calculate the true groundspeed of the suspect vehicle.

Vehicle speed can also be measured and enforced by a stopwatch or GPS mapping system in overhead planes.

Every SMD used by law enforcement is tested and certified for accuracy at least once every two years.