Speed Measuring Devices-How They Work
Posted Friday, March 30, 2018 by Andrew Charles Huff
I have represented literally hundreds of people who have been cited for speeding and other traffic violations. Those who have been pulled over for speeding likely know that your vehicle speed was likely recorded by a laser or radar Speed Measuring Device (SMD). Law enforcement agencies use both radar and laser SMDs to enforce speed limits.
How “Speed Enforced By Radar” Really Works
RADAR stands for “Radio Detection And Ranging” and 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.
The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc. We are most familiar with the Doppler effect because of our experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren’s frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source.
The Doppler effect doesn’t just apply to sound. Light also travels using wavelengths. And this is how Troopers measure speed.
How do Troopers Use Radar for Speed Testing?
When measuring for speed, a Trooper 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 tells the Trooper the object’s estimated speed.Law enforcement officials typically measure speed in three different scenarios: while stationary on the side of a roadway, while in a moving motor vehicle, or from an overhead aircraft.
While stationary, if the Officer or Trooper suspects a driver is traveling faster than the posted speed limit, they will simply aim an SMD at the vehicle to detect the actual speed and confirm their suspicion.If an SMD is mounted in a moving motor vehicle, 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.
