Maryland and Virginia will begin measuring highway congestion early next year by tracking motorists talking on cellular telephones as they drive the Capital Beltway, an experiment that could revolutionize rush-hour traffic reporting.

The two states will test the technology on a 15-mile stretch of the Beltway between the Springfield interchange and Route 5. If the year-long trial is successful, traffic researchers say, the system eventually could be used to provide commuters with more timely and accurate travel information over nearly the entire road network--wherever motorists are speaking on their cell phones.

"There's a lot of eyes on this idea around the United States. A lot of people in my position in other states will be watching to see if this works," said Mike Zezeski, who directs traffic information services for the Maryland State Highway Administration. He said he was optimistic that officials would determine by this summer that the information is reliable.

The most daunting challenge may not be technical but rather convincing the public that the initiative will not violate their privacy. Sensitive that fears of Big Brother could derail the experiment, state officials and their consultants have stressed that they will not be able to monitor phone calls or identify callers. The purpose is simply to follow the energy pattern generated by thousands of cell phones, they said.

"The best we can discern is a bunch of anonymous blobs moving around," said David J. Lovell, a civil engineering professor at the University of Maryland and a consultant on the project. "Nobody at the State Highway Administration will be able to sit down and track the travel pattern of their wife's suspected lover."

Traffic engineers have long complained that their current system of using road sensors to monitor traffic is too expensive and unable to generate comprehensive, reliable information about a daily experience so intimately shared by thousands of drivers.

Although state transportation officials previously had considered tracking cell phone users as traffic probes, it is only within the last year that technical developments and the spread of wireless phones have made this realistic, experts said. The number of cellular subscribers nationwide is now estimated to exceed 80 million.

The initiative was born of a mandate from the Federal Communications Commission that cellular companies offer by 2001 the capability of providing a user's location as soon as an emergency call is placed to 911. U.S. Wireless Corp. of San Ramon, Calif., developed a system to meet this requirement and has tested it in California and Montana. The Beltway project would be the first effort to extend this technology to traffic.

Under a $750,000 contract, U.S. Wireless will install computer equipment on existing cellular towers to register the changing location of cell phone users along the southern segment of the Beltway. The information collected at each tower will then be forwarded to a central control center.

The most difficult technical task will be to assemble the information from separate towers in a uniform fashion because they are operated by different cellular companies, said Paul Najarian, telecommunications director for the Intelligent Transportation Society of America, which includes business, government and academic specialists in high-tech transportation projects.

But Najarian said he was confident the technology would succeed in a heavily developed area such as Washington because the presence of so many buildings makes it easier to map the cell phone signals. "In an area like the Capital Beltway, it'll definitely work," he said.

Hundreds of thousands of commuters on the region's roads are believed to carry cell phones. Experts said that even if only 1 to 2 percent of those motorists are using their phones at a given moment, this still would represent thousands of points of information--more than enough to measure traffic.

Moreover, richer information would be available in congested areas because cell phone use increases as traffic slows: Commuters call their spouses and bosses to say they're running late, conduct business, check their voice-mail or simply pass the time. Even short calls represent useful data, though experts suspect the information gleaned from longer calls will prove better.

State officials said they recognize that talking on a cell phone can be hazardous while driving. "This pilot project is not designed to encourage or condone cell phone use while driving. It takes advantage of a societal trend that is already occurring," said Suzanne Bond, spokesman for the Maryland highway administration. She said the state would continue to urge motorists to pull over if they cannot conduct their conversations safely.

The initial readings may be posted on the Web and made available to radio traffic reporters as early as this winter, and the system could be expanded across both Maryland and Virginia over the next five years, according to state officials.

The real-time information from cell phone signals could eventually be available for in-car computers, traffic specialists said. "Ten years from now, you might be able to get into a vehicle, punch in your destination, and it will tell you which is the fastest route to take," Zezeski said.

Current traffic estimates are largely based on road sensors embedded in the pavement. The problem with those devices is that they are installed only on main highways and are able to measure traffic just at their immediate location, leaving much of the road network in a vast blind spot, traffic engineers say.

Moreover, officials in both Maryland and Virginia report that the combined abuse of freezing pavements and heavy truck traffic often puts sensors out of service, and repairs are difficult because they require closing lanes. Maryland officials estimated that up to 40 percent of their sensors are down at any time.

The breadth of information generated by tracking cell phone use eventually could be used to ease congestion by better calibrating ramp meters and linking traffic signals with actual conditions, transportation experts say.

The data also could make electronic signs more effective. For instance, Lovell said, if officials want to urge some traffic off a crowded highway onto a parallel road without forcing all cars onto the alternate route, they need to know how strongly to word the electronic sign so that some motorists follow the detour advice and others ignore it. The real-time data collected on both roads could help officials judge what message has the desired effect.

Brian L. Smith, a civil engineering professor at the University of Virginia, said the information could be tapped to make short-term traffic forecasts. For example, by measuring the changing speeds of traffic on southbound Interstate 395 near the Pentagon, officials might be able to predict backups that would arise 15 minutes later at the Springfield interchange and post electronic messages to give commuters advice on alternate routes.

Many traffic researchers in the United States and in Europe are optimistic that such detailed information could ultimately make it possible to forecast backups an hour before they happen.

"This could potentially provide a lot of data at a really low cost," said Smith, who advises the Virginia Department of Transportation. "If this works, it's going to take off really quickly."

Tracking Traffic by Telephone

Maryland and Virginia will begin an experiment next year to measure traffic flow on a segment of the Capital Beltway by tracking cellular telephone signals. The $750,000, one-year contract with U.S. Wireless Corp. will provide real-time speed and congestion data to be used for traffic management and planning.

How it works:

The tracking system is based on the notion that a large number of drivers use cellular phones en route to their destinations.

* A driver makes a call using a cellular telephone.

* The radio signals emitted from the phone are picked up by cellular towers.

* The towers in the test area are equipped with a computerized tracking system that collects cellular phone signals as they reach the tower.

* This signal tells the system that a car at a specific location is placing a call. The signal is tracked as long as the call continues.

* The system calculates the speed of the car based on how long the call lasted and how far the car travelled.

* By tracking calls from as few as 1 to 2 percent of the drivers, a clear picture of the driving conditions on that road segment can be made.

SOURCE: U.S. Wireless Corp.

CAPTION: Cars head north on Interstate 95 at the Springfield interchange, near where phone usage will be tracked.