Police radar transmits radio waves at a fixed frequency. Some of the beam reflects from a target vehicle and returns to the radar. If the target is moving, a slight change in frequency occurs (Doppler shift). The radar does the heavy math and calculates target speed.

How Police Radar is Used

Two types of radar are used—stationary and moving. Stationary radar must be used from a static site, typically a patrol car parked at roadside, sometimes a motorcycle. The most common type of stationary-mode radar is the hand-held model that looks like an oversized pistol.

Moving radar allows an officer to clock vehicles while driving on patrol. It can clock oncoming vehicles and if it has dual antennas, a departing car can also be clocked from behind, after it's passed the rolling cruiser.

Most moving radar has discreet components—front antenna, sometimes a rear-facing one in back—plus a control/display module and remote control. Designed for permanent mounting in a vehicle, occasionally waterproof versions are installed on motorcycles.

With one exception, state highway patrols prefer moving radar. In Pennsylvania, the law permits only the State Police to use radar and then only in stationary mode. Hand-held radar, usually K-band Decatur models, have been the mainstay of the Pennsylvania State Police for years.

Without access to radar, many local police and sheriff's departments in the state use time/distance computers like VASCAR. These time a vehicle between two reference points and display its average speed. The technology works fine, but it's more labor-intensive than just pointing a radar gun toward the target.

Radar can transmit continuously or be placed on hold, ready to fire but not transmitting. With no signal present there's nothing to detect, neutralizing a radar detector. When a target draws near, a button-press triggers the radar and a speed appears almost instantly.

Officer preference dictates the choice. Some let it run constantly, content to get fewer customers except for the brain-dead. Aggressive officers are more likely to use instant-on, hoping to outwit drivers packing radar detectors.

Radar Frequencies

North American radar can operate on X, K or Ka band. About one percent of the 100,000-odd radar in service nationwide use X band, 15 percent use K band and the rest operate on Ka band.

X band radar has nearly disappeared except in one holdout state, Ohio, whose highway patrol prefers X band and its wider beam. But others aren't quite so particular and everywhere else there's been a wholesale shift to Ka band.

How Far Away Can it Get Me?

Even in a worst-case situation like our Hill/Curve test site, good detectors give half a mile of warning, plenty of time to react. Skeptics aside, the right radar detector can more than even-up the odds.

On a busy highway, a moving-mode clock usually occurs at less than 1,000 feet, often much closer. Radar merely displays a speed—it's the officer's responsibility to determine which vehicle the radar is looking at. Legally at least, the officer is required to establish a tracking history before taking action.

Done right, this can take several seconds, more than enough time for an alert, radar detector-equipped driver to spike the brakes. Learn more about radar range.

Radar Detectors

A high-quality radar detector can usually spot radar from miles away. These are expensive—$250 and up—but one can pay for itself in a day. A cheap detector occasionally works okay when conditions are perfect. But don't be shocked if sometimes it doesn't utter a peep before you see flashing lights in the mirror.

Some dismiss detectors as mere ticket-notification devices, claiming that instant-on radar can beat it. Sometimes that's true—get hit at point-blank range by instant-on radar and the cop usually wins.

But a quality radar detector can spot the same radar from miles away as the officer targets other vehicles.

Features to Look For

If you commute or drive anywhere near a city, a model with GPS is the best bet. Satellite technology lets it lock out nuisance signals like automatic door openers. Without GPS it'll alert every time you pass within half a mile of a Walmart. View the best GPS-enabled detectors.

Something else to look for: Good K-band filtering. Without it, expect to be plagued by endless false alarms. A lot of these can be blamed on Blind Spot Monitoring (BSM) systems on cars. Most use radar, warning the driver when a lane change is risky.

Some police radar guns use the same K-band frequency, but their number is tiny compared to millions of BSM radar. The problem continues to worsen as BSM radar appears on an increasing number of cars.

For example, of the 93 K-band alerts we logged during one 1,300-mile road trip recently, 99 percent were caused by BSM radar. Four alerts were from roadside radar message trailers. None of them were police radar guns.