General Motors has equipped a few Cadillacs with a prototype system that watches out for the other cars on the road and can slam on the brakes to prevent a rear-end collision, Automotive News reports. The CTS and STS vehicles demonstrated the future of vehicle-to-vehicle electronic technology during the Convergence conference in Detroit in mid-October. Using a combination of global positioning navigation devices, short-distance radio communication and input from existing in-vehicle sensors, the vehicle-to-vehicle -- or V2V -- cars can tell each other where they are and calculate whether danger threatens. GM drivers demonstrated by positioning a Cadillac at the end of a long straightaway set up in a parking lot. Another car then accelerated toward the rear of the stopped one. Passengers in the approaching car saw an instrument panel warning appear as the distance to the stopped car shortened. First a green car-shaped emblem was displayed on the in-car screen. Then a yellow, larger version popped up. As the distance closed further, the brake and backup lights of the stopped car began to flash automatically. At the same time, a red warning appeared inside the moving car to signal danger. At what seemed to be the last moment, automatic braking kicked in and stopped the moving car quickly and safely. GM officials say the system is potentially far less expensive than equipping individual cars with radar sensors dedicated to blind-spot detection. The system, updated 10 times per second and active up to about 500 feet away, can give plenty of warning to prevent a driver from merging into an occupied lane and could provide other traffic control measures in the future, reducing delays and fuel waste at stoplights as well as the cost of roadway infrastructure, according to GM. Drivers said the cars communicate over a 5.9-gigahertz bandwidth allocated by Congress in 1999 for vehicle safety systems. The same V2V system is also used as a blind-spot warning system by the demonstration vehicles. The system is effective with a surprisingly small number of cars equipped with V2V capability -- 5 to 10 percent, or about 23 million passenger vehicles in the United States. But minimal penetration would lead equipped cars to form mini-networks of connected vehicles that, in turn, could regulate the behavior of many other drivers close to them.