Nineteen-ninety-four was a harrowing year for then-Seattle resident Demetrius Thompson. First he was rear-ended. Then he was struck while walking across an avenue by a car running a red light. Fortunately, neither accident was serious. But in both cases the driver had been talking on a cell phone -- and that gave Thompson an idea.

Twelve years later, he's finally ready to show that idea to the world: it's a system that uses the Global Positioning System (GPS) chips lodged inside many cell phones to track a vehicle's coordinates. Whenever a driver who's talking on a phone closes to within 100 meters of a stoplight, the system interrupts his or her conversation with a loud chirp -- providing a not-so-gentle reminder to slow down. Thompson (now living in Los Angeles) has demonstrated a prototype to city engineers and set up a company, Global Mobile Alert, to market the idea to cellular carriers, who could offer the warning system as part of a growing array of data services available to mobile subscribers.

"It's about saving lives," says Thompson, who has worked fulltime on the project since 1994. "If you're driving and talking on your cell phone, you're not really paying attention to what's going on in front of you. But when you hear that mobile alert chirp, it's going to bring you back to reality and give you a good 7 to 10 seconds before you will be at an intersection."

Thompson's demonstrations have been limited so far to the West Hollywood neighborhood of Los Angeles. But if the idea attracts interest, it will join a number of other traffic safety technologies being tested in cities across the country. In Michigan, for example, Motorola has experimented with a wireless system that would warn drivers when vehicles ahead of them are slamming on the brakes (see "Wireless Highway," March/April 2006). But such experiments usually involve adding new equipment to cars, such as accelerometers, wireless transmitters, and GPS receivers -- and they wouldn't work in practice unless all vehicles were similarly equipped.

The only requirement for using Thompson's system, however, is a GPS-capable cell phone. The phone compares signals from GPS satellites to determine the vehicle's location, direction, and speed, and transmits that information over the cellular data network to a computer server built by Global Mobile Alert. The server contains a database with the exact latitude and longitude of all stoplights and other traffic hazards in the driver's area. If the server calculates that the cell-phone user's vehicle is nearing one of those positions, it sends a chirp resembling the cuckoo-clock sound played by some pedestrian-crossing systems for the benefit of the visually impaired.

Jim Carlin, a strategic account manager at market research firm Frost & Sullivan, has reviewed Global Mobile Alert's technology, and says it would be natural extension of cellular telephony's original purpose. "If you look back 10 or 15 years when cellular was still growing, one of the key selling points was that a cell phone would help you get aid if you are involved in an accident or a crime," Carlin says. "It's a way of enhancing safety when you're out there on the road. By going back to those roots, Thompson may be hitting a sweet spot."