Although federal highway safety officials wanted to keep it secret, speeders on the Capital Beltway should know that beginning tomorrow their cars will be photographed by a system that ultimately could be handing out tickets.

During a summer-long experiment, no speeding tickets will be issued as a result of the photos. But the test on the 23-mile Virginia stretch of the Beltway, the first federal highway under the camera's eye, could lead to a permanent system of so-called photo radar there.

The system uses radar to determine when a vehicle exceeding the speed limit passes by and a camera snaps a picture of the car's license plate. The car's owner -- who may not necessarily be the driver -- gets a ticket by mail.

Word of the test leaked out despite an attempt by the National Highway Traffic Safety Administration, which is financing the experiment, to keep it under wraps. Virginia State Police had planned a news conference to announce the experiment, according to sources, but the safety agency prevented it, fearing the publicity would backfire.

"It's only a test. They didn't want to alarm the public," said Thomas Rametta, of the Metropolitan Washington Council of Governments. He said federal officials thought "if there's a lot of publicity, people might be upset. You know, that 'Big Brother is watching us.' "

Safety agency officials could not be reached for comment.

Virginia and Maryland law enforcement officials have been advocating photo radar for several months. Speed enforcement is difficult on the Beltway, they say, because it is dangerous for troopers to pull over violators in heavy traffic. They also say vehicles stopped on the highway shoulder are safety hazards.

Critics say photo radar is government snooping into citizens' lives. A spokeswoman for the American Civil Liberties Union predicted the technology would result in a rash of lawsuits.

The nation's largest motorists organization, the American Automobile Association, has objected to the state's using cameras and radar guns to keep watch on drivers.

Giffen B. Nickol, of Bel Air, Md., a member of the Maryland chapter of Citizens for Rational Traffic Laws, said photo radar "takes us yet another step closer to an Orwellian world in which our government routinely robs us of our liberty in order to protect us from some alleged harm."

At first, Virginia state troopers will test only the photo radar equipment, supplied by several manufacturers, to see which type works best.

The devices generally are installed in vehicles, on roadside tripods or on overpasses. A trooper points the camera at a Beltway lane, sets it at the speed limit and loads the camera. Pictures of a speeding vehicle's rear license plate are taken, with the time, location and speed noted on the film.

The trial period is expected to end in September, and then officials will recommend whether to continue exploring the system.

The Virginia and Maryland legislatures would have to approve legislation for the speeding tickets to have the force of law, officials said. Maryland State Police are participating in the current study, but Virginia is taking the lead in installing the cameras for the initial test.

In Pasadena, Calif., one of two U.S. cities using photo radar, the number of speeding drivers has dropped by 30 percent, officials said. Such systems are widely used in Europe.