THE WONDERS of modern technology never cease: For $750,000, Maryland and Virginia soon will try out a new way to catch up with stalled traffic. A system coming early next year, to detect congestion, will track Beltway motorists talking on cell phones. The telltale talk won't be the actual conversations--"I've been stuck on this (expletive deleted) spot for three hours!"--but special phone signals that can be used to calculate the speed, or lack of it, of the vehicles. Thus, the experts say, privacy will not be violated, at least no more than it already may be on these phones.

Many traffic researchers in this country and in Europe are said to be optimistic that the detailed information gleaned by the system ultimately will make it possible to forecast backups an hour before they happen. As it stands, most Beltway motorists don't know exactly where or when they will have to hit the brakes and idle, only that they will. The new system will take away the surprise element by telling them even before they leave home where misery awaits. This way, smart motorists can try to beat the system by taking side streets with their phones turned off.

State officials acknowledge that talking on a cell phone can be hazardous while driving--which has led some state lawmakers to propose banning phone use by motorists in motion. We have supported that idea, though here motion occurs only occasionally. Perhaps the experiment will lead to new solutions. In any event, the longer that regional officials fail to build desperately needed additional roads and bridges and to improve mass transit, the easier it will be to spot congestion without any phone calls.