Abstract: A vehicle safety device includes a vehicle mounted transceiver for transmitting an inquiry or “challenge” code to an operator-carried transponder which processes the challenge code according to a secret algorithm (which is present also in the vehicle unit), and transmits a response code to the vehicle. The transponder also includes a processor for superimposing on the response code additional information which must be present in order for the vehicle unit to recognize the response as valid. In a preferred embodiment, the bits constituting the response code are transmitted at differing power levels which depend on the data content of the response code.