Fairfax County officials have moved on two fronts to address growing concerns about school bus safety: Seventy new buses equipped with seat belts are arriving and county officials announced yesterday that they will crack down on drivers who illegally pass school buses or speed through school zones.

Police spokesman Warren R. Carmichael said yesterday the problem of school bus safety may have grown in part because of the increasing number of cars in the county. "I suspect the increased congestion has resulted in some drivers being frustrated at having to slow down for anything," he said.

Also yesterday, police for the second time this fall charged a county school bus driver with reckless driving in connection with an accident in which the bus hit a car in Springfield. There were no injuries.

The new school buses with seat belts will begin regular runs in the county in January in an experiment to see whether they improve safety, officials said. The first buses have arrived and the rest will be delivered in the next month, said Max Skidmore, acting assistant superintendent for general services.

The 70 new 64-passenger lap-belted buses, plus three 54-passenger buses with lifts and belts for handicapped children, cost $2.4 million, partly paid for with $100,000 from the county Board of Supervisors. Many school systems already use seat belts on buses for handicapped children, but Fairfax is among the first to try them for standard school buses.

Opinion on the usefulness of seat belts is mixed. Advocates, including parent groups, contend seat belts are safer and that they encourage children to use belts in their family cars. But the school bus industry and some federal officials who oppose them contend they do not increase safety and are expensive. Fairfax County spent $1,307 more per bus for the belts.

The buses will raise the size of the Fairfax fleet, already the largest in the nation, to 979, said Skidmore.

Officials said after yesterday's bus accident that the two dozen children aboard were not injured, but the car's driver, Judith E. Beach, 46, of 7922 Journey Lane, Springfield, was taken to Fairfax Hospital where she was treated and released. Police did not know whether she was wearing a seat belt.

The slightly damaged bus continued on to Forestdale Elementary School after the accident.

Police spokesman Dave Russell said the bus driver, Patricia A. Minner, 23, of 8405 Lazy Creek Ct., Springfield, ran a stop sign and hit the car at Northumberland Road and Edinburgh Drive.

Police said Minner had a valid driver's license. She has been suspended with pay from her job pending final action on the reckless driving charge, and will be dismissed if convicted on that charge, school spokeswoman Dolores Bohen said. Minner began work with Fairfax County in September, Bohen said.

The first driver arrested this fall, Edward Hall, 23, of 1450 Q St. NW, was charged after an Oct. 24 crash in which he and two children were hospitalized. When police later learned he had a suspended D.C. driver's license, Hall was charged with providing false information when he applied for his Virginia driver's license. Hall has been suspended without pay pending a hearing.

Fairfax County police said their school bus safety campaign will begin on Dec. 2 to focus on ticketing drivers who pass school buses displaying flashing red lights and drivers who speed over the 25 mph limit in school zones. It is illegal to pass a bus with flashing lights unless driving in the opposite direction on a divided highway.