Before you purchase a radar detector, it's important to understand the different kinds of radar used by the police. Police radar guns work by emitting a microwave pulse to measure the speed of a moving vehicle; radar detectors work by sending out a signal that mixes with this pulse to produce a lower, more easily detected frequency.
X-band radar (10.5 to 10.55 GHz) used to be the most popular gun used, but it is now all but obsolete (sources say police in New Jersey and Ohio still use it regularly, though). X-band is also the frequency that causes the most false alarms. That's because many supermarket automatic doors, garage-door openers and motion detectors run on the same frequency. Even inexpensive radar detectors often have a city mode that lets you downgrade or disable a detector's sensitivity to X-band, thereby cutting down on the number of false alarms from sources other than law enforcement.
The majority of radar guns operate on the K-band (24.05 to 24.25 GHz) or Ka-band (33.4 to 36 GHz) frequency. K-band and Ka-band guns are especially tricky because they can be turned on instantly. If an officer chooses your car, it's almost impossible to be warned ahead of time. However, good detectors can provide a warning if a radar gun is targeting vehicles ahead of yours, giving you enough time to slow down.
Law enforcement has found a new way to stack the deck with POP modes on a few radar guns. POP modes allow officers to send out signals in bursts too short to be picked up by many radar detectors. The catch is that speeders caught using POP modes can't be ticketed because the law requires a radar gun to lock onto a vehicle. However, if law enforcement officers detect a speeding vehicle using POP mode, they can then flip the gun into constant-on mode and get a lock on a vehicle. POP poses no problem for any radar detector in tests, but it's not widely used by police anyway.
Laser guns are increasingly popular. Police particularly like to use them on crowded roads because laser light beams are so tight, they can easily pick a speeding car out of a crowd. Another positive for police is that, generally speaking, laser guns give off little advance warning, unlike radar, which puts out a relatively broad signal that can be detected miles away. That means that once a laser detector sounds its alert, it's often aimed squarely at you.
Laser guns do have some downsides for police, however. Because the laser must reflect back to the gun off of a flat surface, the police have to get a good aim, usually at your front or back license plate or headlights. In addition, laser guns have to be used from a stationary position, giving you some opportunity to visually spot the speed trap. And if the officer turns on the laser gun too soon, a laser detector may give you enough warning that you can slow down before you get within ticketing range.
Most radar detectors can also detect laser guns, but experts say a laser detector alone is of limited use. Instead, they recommend coupling the detector with a laser jammer (which, unlike radar jammers, remain legal in most states) or another countermeasure such as an anti-reflective coating on your headlights or license plates.
Finally, some police departments use a device known as a radar-detector detector (RDD) to alert them to vehicles equipped with radar detectors. For passenger cars, using a radar detector is legal everywhere in the U.S., except Virginia, Washington, D.C. and on military bases. However, they are illegal in big commercial trucks and in much of Canada. A few detectors can cover up their telltale signal leakages, proving invisible to all RDDs in tests: The top-rated Escort RedLine (Est. $500) , the similar Beltronics STi Magnum (Est. $480) and the pricey custom-installed Beltronics STi-R Plus (Est. $1,300) are examples.
It's also important to realize that no radar detector will report every single radar encounter. There are too many variables, such as terrain, angle of the gun to the car, interference from other traffic, etc. Although certain models do better than others, no radar detector will spot 100 percent of radar threats 100 percent of the time. If you want to be absolutely sure of never receiving a speeding ticket, the only sure defense is to not exceed the posted speed limit.