This invention is about a system that monitors driving habits, to be used by automotive insurance providers in safe-driving discount programs. The idea eliminates any of the privacy concerns associated with data loggers that are typically used in such programs, and should thus find a much more widespread acceptance.

In this system, low-power radio transmitters are installed on speed-limit signs to emit at precisely the same frequency. A receiver installed in a car will pick up these signals at slightly shifted frequencies due to the Doppler effect: a lower frequency from behind, and a higher frequency from ahead. The receiver determines its travelling speed from the difference frequency (beat note) between these two signals. Encoded in them are also data packets that inform the receiver of the local speed limit, which may even depend on the date or the time of day. If the receiver in a car detects a speed in excess of the local limit, it emits a warning tone and records the incident in its memory.

To receive an insurance discount, the driver occasionally reads from the receiver a code that contains an ID, and the number of severity of speeding incidents (if any). The code contains only a few (10 to 20) characters, and is thus, obviously to the driver, too short to contain any details about times or locations of the speeding incidents, or any other routing data.

This will mitigate privacy concerns that limit the acceptance of data loggers issued by insurances for their safe-driving discount programs.

This should be interesting to all participating parties:
Insurances to lower their payouts, customers to get a discount, cities to improve traffic safety, and manufacturers of RF equipment to sell this technology.

At radio frequencies, the Doppler shift corresponding to a speed of a few miles per hour is only a few Hertz. The transmitters at the two ends of a speed-limit zone must match each other's frequency to that level of precision, but the absolute frequency does not need to be particularly stable. This requirement can be met by designating one transmitter the master, while the other transmitter simply receives and re-transmits that signal.
Because the transmitters are not moving relative to each other, the re-transmission occurs at exactly the frequency determined by the master. In spacecraft communications, this principle is known as a coherent radio link. The receiver in the car does not need to be particularly frequency-selective because it only determines the difference of the frequencies from ahead and behind.

The same system can also be used to enforce stopping for a school bus that is loading or unloading passengers. For this application, a transmitter on the bus simply sets up a temporary speed-limit zone of zero.