I would think such a device would be illegal, since it would also interfere with law enforcement's radar guns, giving false readings. Otherwise, the police would be using them, much cheaper than manned patrol. Slamming on the brakes? Get real, everyone should know that brake lights are a tip off that you know you are speeding, and using a radar detector, to avoid a ticket, and won't get any sympathy. You'll get 'rear-ended' alright, in traffic court...