Abstract: A system for transmitting messages, such as safety hazard warning messages, to vehicles causes alerts to be generated in conventional radar detectors and more detailed messages to be provided to drivers with message capable radar receivers. The transmitted radar signal is modulated in frequency or phase or swept in frequency in different directions or at different rates to define the logical state of each bit of the message. A continuous wave (CW) maker signal may also be transmitted to assure detection by a conventional police radar detector and to allow a receiver capable of decoding messages to make adjustments that center an intermediate frequency (IF) generated from the received signal in the receiver IF passband. A message capable receiver decodes the modulated signal to communicate to the vehicle operator the contents of the transmitted message or a stored message corresponding to a transmitted code.