1. A method of announcing a presence of a vehicle operated by a driver, the method comprising: determining a state of mind or physical state of the driver in real time; determining whether a driver is impaired based upon the determined state of mind or physical state of the driver; when the driver is determined to be impaired: generating noise, audible to at least pedestrians that are proximate to the vehicle when it is being driven, characteristics of the generated noise being selected from a plurality of different characteristics responsive to a determined state of mind or determined physical state of the driver; sampling ambient noise from electrical signals received from a microphone; determining a frequency spectrum of ambient noise signals; identifying at least one portion of the ambient noise frequency spectrum, where the ambient noise is lowest relative to amplitudes of other component frequency signals; generating at least one audio frequency signal component, which is in the at least one portion of the ambient noise frequency spectrum where the ambient noise is lowest relative to amplitudes of other component frequency signals; and outputting the at least one audio frequency signal component that is in the at least one portion of the ambient noise frequency spectrum where the ambient noise is lowest relative to amplitudes of other component frequency signals.