Under ideal conditions, most police radars have an accuracy of approximately ± 1 mph. The microwave radar in mobile mode also measures the speed of the patrol vehicle with an accuracy of approximately ± 1 mph. The accuracy of the target vehicle in motion mode is ±2 mph. Some microwave and laser radars specify accuracy based on a percentage of the vehicle's speed.
Mobile radar allows an officer to monitor vehicles while driving on patrol. It can time oncoming vehicles and, if it has dual antennas, a departing car can also be timed from behind, after the rolling cruiser has passed. The radar simply shows a speed that it is the officer's responsibility to determine which vehicle the radar is looking at. If you're looking for a radar detector, you'll want to make sure that the detector's range is at least six times greater than the capture area of a police radar gun.
Think of a radar beam as a cone, narrowed at the radar antenna and widening as it heads toward the horizon. The most common sources are vibrating or rotating signals near the road; fan blades that move in or out of the patrol car (air conditioner, heater, defroster, or engine fan); another moving vehicle that reflects radar waves better than the target vehicle; and multiple targets in the main radar that cause multiple reflections of almost the same intensity and cause the screen to read, high, low or completely blank. Because of this report, federal regulations were instituted to include a speaker in all police radar guns so that the officer could monitor the droppler's radar tone to distinguish a false reading from a true reading. The typical small sedan didn't appear on the radar until it was less than 365 meters from the antenna, but the same radar unit was fixed on a Ford 9000 semi-trailer more than 2.3 km away.
Some police radar guns use the same K-band frequency, but their number is small compared to millions of BSM radars. Police radar is as error-prone today as ever, particularly with the widespread use of radar in instant-on mode. Radar detectors are little more than radio receivers tuned to the same frequencies used by police radars. Although metal reflects radar beams better than most surfaces, virtually any material will reflect radar waves to some extent.
Most radar errors are the result of radar operation under real conditions, which are often not ideal. In the first instance, the reading was caused by the radar antenna panning, and in the second, the radar unit was measuring the fan motor in the patrol car. But a quality radar detector can detect the same radar from miles away, as the officer points at other vehicles. Yes, in fact, being able to visually estimate the speed of a vehicle without the use of a radar gun is part of several radar and laser certification programs and courts have ruled that a trained officer's visual speed estimation is admissible in court.
Many of the detectors available on the market have sensitivity control that can be adjusted to offer the best compromise between trying to detect even weak and distant police radar signals and trying to filter out off-frequency signals that come from sources other than police radar.