The bus-car collision that killed three people on Interstate 95 near Beltsville last April was probably caused because the bus driver was following the car too closely or failed to see it stop, the National Transportation Safety Board concluded yesterday.

The board urged Maryland to consider speeding up planned safety improvements to the I-95/Capital Beltway interchange. The accident occurred during a routine traffic backup at rush hour a mile from the interchange.

The probable cause of the 5:55 p.m. accident was "the failure of the bus driver to maintain a safe stopping distance between the bus and the automobile ahead as traffic ahead slowed and came to a stop," the board's report stated.

Investigators reported that the Trailways bus, traveling south on I-95 from New York, struck the rear of a stationary 1981 Chevette at about 45 mph. The car had stopped in the right lane about a mile north of the Beltway due to the rush-hour congestion on the ramp leading onto westbound lanes of the Beltway.

The impact set off a chain-reaction collision that damaged or destroyed three other cars. The Chevette and a 1976 Dodge Colt caught fire immediately, with flames engulfing the bus after its driver and 34 passengers scurried to safety through the door and emergency exits.

The Chevette's three occupants -- a 46-year-old woman from Severn, Md., her 13-year-old son and another boy -- were trapped in the car and died. Occupants of the three other cars suffered minor injuries.

The board found no evidence that the driver, Harry Lee Patterson, 46, of Laurel, was physically impaired at the time of the accident. He told investigators he believed he was about 100 feet behind a car when the traffic stopped. The board concluded that such a distance was insufficient for the bus to stop from its initial speed of about 55 mph. If the distance was greater, the board said, the driver probably did not immediately perceive that the Chevette had stopped and was late in applying his brakes.

Skid marks left by the bus' tires began only 42 feet from the point of impact.

Police have not filed charges against Patterson, who frequently drove buses on the New York-Washington route, in connection with the accident. But the board's report disclosed that in 1980 he received a ticket for tailgating while driving a bus on I-95.

The brakes on the 40-foot, 7-year-old Trailways bus had been improperly adjusted before the crash, investigators found, impairing the vehicle's ability to stop. However, the board concluded that this fault was not a factor in the accident.

Two, and possibly three, of the Chevette's passengers might have survived, investigators told the board, if their car had not caught fire. Its gasoline tank did not rupture, it was found, but the tank's filler pipe was torn away, leaving a two-inch hole through which fuel spilled.

Similarly, the Dodge Colt's filler pipe was dislodged on impact, allowing fuel to spill out. In both cars the fuel ignited.

Federal standards concerning gas tank design applied to the Chevette but not the Colt, which was built before the regulations went into effect, the board reported. In any case, the rules assume impact speeds of no more than 30 mph between vehicles of roughly equal weight, the board stated. The bus struck the Chevette at about 45 mph.

The board rejected by a 2-2 vote a proposal to ask the Motor Vehicle Manufacturers Association to encourage member firms to design gasoline tanks so that major spills would not occur if filler pipes come off.

The board also turned down a staff proposal to recommend that signs be posted along the highway to warn motorists to watch for stopped vehicles. The board noted that the driver was familiar with the road and signs were already in place warning that I-95 ended ahead.