Does a police officer's radar gun work accurately when his/her cruiser is in motion?

Question

I have heard a rumor that if a cop runs radar on your vehicle while he/she is also in motion that it doesn't give an accurate reading. I have also heard the opposite is true. I ask because, by trade, I do deliveries in my home state, oftentimes travelling upwards of 400 miles a shift (8 hrs) and so I am forced to speed. A couple days ago I was doing about ten over the speed limit on a highway and a sheriff passed me going the opposite direction. He/she immediately u-turned (there was little traffic) and was about a quarter mile or less behind me. A pickup in front of me was doing the speed limit so I passed him and started booking it about 90 mph hoping the sheriff would give up. He/she never pulled me over or turned on their lights, but I was wondering if I was radared accurately or if the sheriff just "guessed" I was going significantly faster and was trying to spook me into slowing down. Answers from law enforcement officers would be appreciated, or any civilians who have had a similar experience.

Answer 1

A radar beam is similar to the beam of a flashlight as it spreads out as the distance from the signal origin increases. The signal then bounces off objects in the path of the beam and are reflected back to the gun. The gun uses the doppler effect to calculate the speed of the object in the beam's path. Using a comparison of frequency shift between received images instead of the frequency shift between sent and received frequencies creates what is known as moving radar, the radar must be stationary to measure speed.

All bands of radar work the same way; they simply operate on different frequencies. X band guns are becoming less common due to the fact the beam is strong and easily detectable. Also, most automatic doors utilize radio waves on X band and can possibly affect the readings of police radar. As a result K band and Ka band are most commonly used by police agencies.

Traffic radar comes in many models. There are hand held, stationary and moving radar instruments. Hand held units are mostly battery powered, and for the most part are used as stationary speed enforcement tools. Stationary radar is mounted in police vehicles, and may have one or two antennas. These are employed when the vehicle is parked. Moving radar is employed, as the name implies, when the police vehicle is in motion. These devices are very sophisticated, able to track vehicles approaching and receding both in front of and behind the patrol vehicle. They can also track the fastest vehicle in the selected radar beam, front or rear.

Answer 2

Radar, moving or stationary, can be accurate to plus or minus 1 mile per hour.  

In order for radar to be most accurate, the target has to be traveling directly at, or directly away from, the radar unit. If there is any angle at all, it loses accuracy due to a condition known as a "cosine error". However, this error is always in favor of the motorist.

In your case, I'm guessing the U-turn was because the deputy was dispatched to a call, and the fact you were there was just coincidence.

Answer 3

The 1st answerer gave a good explanation. Majority of the time stationary radar is the best and most reliable method. However, using radar in motion is fairly accurate, as the devices are bolted onto the vehicle's dash and are stable.  

Answer 4

CHP has the best technology, its pretty close to accurate, may be off by about 1-2 miles but you have to put in the consideration that its the speed of our cruiser, and the speed of your rig, also the wind. My sheriffs office we dont have radars because they try to discourage us from doing traffic stops due to the fact thats CHP's job. But no we more than likely wont write you a ticket if we didnt catch your radar due to what evidence to we have that you were actually speeding? trust me it wont hold up in court and youll get out of it. Most of the time though CHP wont pull you over unless your doing over 75 mph and at 75 mph is a coin toss I hope this helped you at all

Answer 5

Many police are looking for the one who is doing 20-30 miles over the limit, not just 5 or 10. also, if the cop hasn't had his radar adjusted within the time frame he is supposed to if you challenged the ticket in court the ticket could be overturned. He may have just turned around for reasons other than you. Who knows? He could have gotten another call or reached the limit of his jurisdiction.

Answer 6

Certain radar guns can still work in motion, both in the same direction and the opposite. Not all police departments can afford that technology though. Also, an officer doesn't need a radar reading to give a speeding ticket. There are other methods of accessing your speed. My guess is the cop's U-turn had nothing to do with you and was just the end of his patrol area.