The problem:
Speeding is a major cause of traffic deaths. According to the U.S. census of 2009, 33,808 traffic fatalities that year were directly attributable to speeding [1]. Some automotive insurances are implementing programs to improve the driving habits of their customers, such as offering discounts for the installation of a data logger in the car. Such programs are finding limited acceptance due to privacy concerns.

Proposed solution:
Here, a passive speed monitor is proposed to alert the driver of speeding that does not emit signals from a car, and does not log driving data. In this system, low-power radio transmitters are installed on speed-limit signs to emit at a precisely fixed 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 traveling speed from the difference frequency (beat note) between these two signals. Encoded in them are also data packets that transmit the local speed limit to the receiver. If the receiver in a car detects a speed in excess of the local limit, it emits a warning tone. No data are recorded, and therefore there is no reason for privacy concerns. This will greatly help acceptance of such a system.

Insurances can offer discounts in exchange for installation of such a receiver. Optionally, for an additional discount level, the receiver can hold information that it emitted a warning for a few minutes. It then becomes impossible to offer an excuse of not having been aware of the speed limit. The insurance company will want to verify that the receiver was actually turned on. For this, the transmitters provide a code for each given time interval (month, quarter, etc.) to be periodically read off and sent to the insurance, but only if it had been powered on during most of that time interval. That code can be a sequence of a few digits that can obviously not hold logged data, so users can be reassured of their privacy.

In order to achieve a sufficient frequency stability, the transmitters receive the emissions from one designated master, and synchronize their emissions to it. The radio signals must be of relatively low power (comparable to garage-door openers) to limit the range of the system to a given speed-limit zone. Adjacent speed-limit zones can use slightly different frequencies to
avoid interferences.

Market opportunity:
1) Such as system will be attractive to communities concerned about
speeding in their neighborhoods and to insurance companies to lower
their risk exposure.
2) Production of the equipment (transmitters and receivers) is a huge
commercial opportunity to electronics equipment manufacturers.

[1] http://www.census.gov/compendia/statab/2012/tables/12s1108.pdf