The radar jammers that really work are banned worldwide. The ones that are sold inexpensively and are just reflectors, don't work and are a waste of money. This device indicates, to police, the presence of a radar detector in a vehicle. The weaker the operating emissions from a detector the more difficult it is for a radar gun to pick up the signal, therefore making it equally difficult for police to detect the detector. In New York, Virginia, and Washington DC, where detectors are illegal for public use, this gives some advantage over the more powerful detectors.
Radar detectors are illegal.
New York and Washington DC have similar statutes. However, if the detector is in the vehicle, but not hooked up, and not accessible to the driver or passengers, the statutes are not in violation. Although drivers should always stay within the posted speed limit, there are places where speed rules just don't seem to make sense to me - Montana backlands, large parts of Nevada, the Dakotas, etc. It's annoying to be caught by police lying in wait to catch you just as the posted speed limit drops by half in the middle of the wilderness for no apparent reason. Radar detecting systems by well-known manufactures can be purchased relatively inexpensively.