The appeal of radar detectors is pretty obvious: no one wants to get caught in a speed trap. Speeding tickets are generally fairly expensive and if you get caught more than once, they can really add up quickly. Detectors are used to alert a driver if there is a radar device being used to monitor the speed they are traveling allowing them to slow down and avoid getting stopped for speeding. It is one of the more convenient driver tools available, but there may be a price for that convenience depending on where you live. In some states it is illegal to have a radar detector in your car which means if you are caught with one, you may end up with a fine much more severe than a fine for speeding.
Detectors detect electro-magnetic emissions from a radar gun used by the police to monitor your speed. One of the major problems with the traditional radar detector, however, is the high number of false alarms that can be triggered by everything from automatic doors at supermarkets or department stores and a number of other devices.
In response to false alarms, many modern detectors also employ GPS technology. These GPS detectors allow you to build an ignore list by programming in locations of frequent false alarms. Once these locations are programmed in, the radar detector will ignore these locations which greatly reduce the amount of false alarms. This is easy to do and does not require the motorist to input any coordinates. Instead, it is done automatically with the push of a button. You can also use these GPS models to store locations that are frequently used as speed traps alerting you to their presence in the future as you approach them. More advanced GPS radar models download the location of speed monitoring cameras as well as red light cameras.
In Virginia, Washington DC and on all military bases, the use of detectors is illegal in all vehicles. While legal in California, Minnesota and New York you cannot mount the device to your windshield as it will be considered an obstruction of vision. Keep in mind; there may be other regulations in your state, so if you intend to purchase a radar detector it may be a good idea to consult with someone who is familiar with current law enforcement policies to make sure you are able to use one in your area.
The general rule, aside from the above exceptions, is that it is illegal to use a radar detector in a commercial vehicle but legal in private vehicles. However, if you live in a state where this is the case and you travel to a state where it is not, you may still be fined if you are caught with one. It is worth noting that detectors are prohibited in all Canadian provinces aside from Alberta, British Columbia, and Saskatchewan.
While detectors are a great tool for helping you avoid speeding tickets, there is really no better method than watching your foot on the gas pedal and obeying the speed limit.