Before you purchase a radar detector, it's important to understand the different kinds of radar used by the police. X-band radar (10.5 to 10.55 GHz) used to be the most popular gun used, but these are now almost totally obsolete. X-band is also the frequency that causes the most false alarms. That's because many supermarket automatic doors, garage door openers and microwave towers run on the same frequency. More expensive radar detectors have a city mode that lets you downgrade or disable a detector's sensitivity to X-band, therefore cutting down on the number of false alarms from sources other than law enforcement.

Right now, the majority of radar guns operate on K-band (24.05 to 24.25 GHz) or Ka-band (33.4 to 36 GHz). Ka-band guns, account for 90% of new radar-gun purchases, and they now are the mostly widely used radar technology. 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, most detectors can provide a warning if a radar gun is targeting vehicles ahead of yours, giving you enough time to slow down.

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. Newer radar guns are licensed to operate in any of three frequencies on the Ka-band (33.8GHz, 34.7GHz and 35.5GHz) and are now the most common type of radar gun in use. Reviewers emphasize Ka-band performance more than X- or K-band, since Ka-band is both harder to detect and increasingly popular. Photo radar also operates on the Ka-band and photographs the license plates of speeding vehicles.

Law enforcement has found a new way to stack the deck with POP modes on Ka-band and a few K-band 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 detects a speeding vehicle using POP mode, he or she can then flip the gun into constant-on mode and get a lock on a vehicle.

The laser gun is also becoming more popular. Laser can use a much smaller beam than radar, and once a laser gun has locked onto you, it's too late. And because laser light beams are so tight, they can easily pick a speeding car out of a crowd. This makes laser guns an increasingly popular choice for congested cities, since officers can single-out specific cars on a crowded roadway. Since the laser must reflect back to the gun off a flat surface, the police have to get a good aim, usually at your front or back license plate or headlights. Detectors have a much tougher time picking up a laser gun than standard radar guns. Laser guns have to be used from a stationary position -- giving you some opportunity to visually spot the speed trap.

Some radar detectors can also detect laser guns, but if the detector does alert to laser, it's already too late. The officer has already locked onto your vehicle. Although many radar detectors advertise their ability to alert to laser, the feature is more or less useless. The only defenses against laser guns are laser jammers and other laser countermeasures.

Finally, some police departments in places that restrict the use of radar detectors use VG-2 or RDD, a device that can detect radar detectors, but nearly all radar detectors are now undetectable by VG-2. The latest detector-detector is Spectre II, which can sniff out all but a few models of radar detectors. However, most drivers don't need to worry about these devices. That's because owning and using a radar detector is perfectly legal in all states except Virginia, Washington, D.C. and military bases. They are also illegal in most parts of Canada.

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. While certain models do better than others, no radar detector will spot 100% of radar threats 100% of the time

Here's what experts say to consider before choosing a radar detector:

• The further away the device can detect radar guns and laser, the easier it will be for you to slow down in time to avoid a ticket.