When the Fiat driver cut across his lane and braked hard, Robbie Crawford saw red. "I was shaking with anger," says the Edinburgh computer consultant. "I really lost my head." But now, several years later, the 29-year-old is starting to view his brush with road rage in a different light—green, as in the color of money.

Crawford's fury sparked the idea for an invention: a device to defuse road rage. How about, he thought, a little electric sign that attaches to the inside of a car's rear windows and that, at the flick of a button, lets the driver signal a message? Voilà! the birth of the Envoy. Powered by the car's cigarette lighter, it flashes a choice of three sentiments—"thanks," "sorry" or "help." (It can- not, he emphasizes, be reprogrammed to flash "rude messages.") "It is just a silly thing that makes people's lives a bit easier that I wanted for myself," says Crawford, who makes his home with wife Sharon, 29, their infant son Samuel and three cars. "I will be happy if it does no more than cool a few tempers on the road."

Since his firm, Urbane Systems, began production in May 1998, the 10-inch by 3-inch plastic devices, which sell in Great Britain for about $65, have accomplished considerably more than that. "Every day we get piles of letters from people, sometimes more than 100," says Crawford. His brainchild has been endorsed by the Royal Automobile Club (the British version of AAA), and he plans to have it in U.S. stores in time for Christmas. "One friend of mine," he says, "was chased for nearly 10 miles up a motorway by someone who wanted to buy one."