A few months ago in Michigan, a sedan, followed by a minivan -- both rigged out with prototype wireless communications equipment and software -- swung onto Halsted Road in Farmington Hills. The driver of the sedan then slammed on his brakes, as if a dog had run in front of his bumper. This is the kind of abrupt move that can cause a rear-end crash, especially when visibility is poor.

But this particular sedan had a computer in its trunk outfitted with a Global Positioning System receiver and a short-range radio. The abrupt brake-jamming registered on the computer, which broadcast a warning and the sedan's GPS location. The minivan, similarly equipped, picked up the warning via special radio frequency, calculated that the sedan's location was just ahead of its own, and warned the driver, sounding a chime and flashing a red light.