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Ask A Cop: Do radar detectors work?

Editor's note: Every week a Lake Oswego police officer answers your questions in this space. Send your questions to reporter Cliff Newell at This email address is being protected from spambots. You need JavaScript enabled to view it. or call 503-636-1281, ext. 105.

"Do radar detectors work? Are they illegal? Have you ever pulled someone over and caught him with one?"

Radar detectors do in fact work, but there are way too many factors to discuss in this short article that relate to their effectiveness.

Think of a radar signal as a beam of light from a flashlight. When you shine the light at an object, your eyes perceive the light reflected from the object. Now imagine yourself as the object being illuminated. You can see the light from the flashlight from a much father distance away than the person with the flashlight could ever hope to see you. That’s because the beam loses energy over distance. While the beam has enough energy to reach you, the reflected light doesn’t have enough energy to travel all the way back to where it started.

Police radar guns “see” a vehicle by transmitting a microwave pulse. Then they make use of the Doppler effect: The frequency of the transmitted pulse is compared to the frequency of the reflected pulse and speed is calculated by using the difference between them.

That’s the idea behind radar detectors. They look for radar “beams” and find them before they can return a strong enough reflection to “illuminate” you.

Most all police radar in use today has a standby mode that prevents the unit from emitting a constant beam. Officers are trained to not activate their radar until they see a vehicle visually traveling above the speed limit. The radar unit will return a speed reading a lot faster than drivers can take their foot off of the accelerator and apply the brakes when their detector goes off; therefore, the officer will already know your speed before you can slow down.

Radar detectors are not illegal to use in the state of Oregon unless you are driving a commercial motor vehicle. Police stop people all of the time that have radar detectors in their cars. Most of time I inform them that by the time their radar detector activates, I have already tracked their speed for several seconds.

— Sgt. Gary deMoss




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