IIRC it's on the device, i.e. stop sign, measures the rate of approach, and
signals the car if it detects it's not going slow enough to stop. Must be
something along the same lines, sonar/radar.

"Leon" wrote in message
...
IIRC the Toyota Avalon and or one of the Lexus vehicles currently have an
option for automatic breaking if you come too close to the car in front of
you on the freeway. Probably works similar to the sensor on my neighbors
Excursion that beeps when you back up too close to an object.