This summer an estimated 500 serious bicycle accidents will occur in the District--half of which will involve cyclists pedaling during rush hour--according to a study by the Washington Area Bicyclists Association (WABA).

In bicycle-vehicle accidents involving adult cyclists, the study notes, motorists are usually responsible. In those involving children or teen-agers, the cyclist is usually responsible.

Most accidents occur in June, July and August "when bicyclists are on the road in full force," says WABA safety expert Carl Modig, who compiled the report from accident statistics gathered by the District police and transportation departments over the last four years.

Most bicycle-vehicle accidents, Modig says, "are preventable. Typically, they're caused by a handful of cyclist or motorist behaviors" such as:

* Improper turns by motorist--13 percent

* Failure of bicyclist to comply with traffic controls--10 percent

* Bicyclist darting out from driveway, alley or sidewalk--9 percent

* Bicyclist riding on wrong side of the street--7 percent

* Motorist running into rear of bicycle--5 percent

* Bicyclist hit by opening door of parked car--4 percent

Three-fourths of accidents occur during the week, and half occur during rush hour. About 13 percent of accidents were blamed on "environmental conditions."

Five bicycle fatalities occurred in the District between 1978 and 1981--about one per year. Three of these happened after dark, and none of the cyclists wore helmets.

"Seventy percent of all fatal bicycle accidents involve skull fractures," the report notes. "Most bicycle helmets are designed to protect against this kind of injury."

Bicylists should "never assume they have been seen by motorists, even when they have the right of way," Modig says. "They should also ride carefully and obey traffic laws."

Last year, 517 serious bicycle accidents involving injury that required medical attention were reported to D.C. police, up slightly from 510 in 1978. But "the actual figure is probably twice that," says WABA, citing a national study which indicates only half of all serious bicycle accidents are reported.

The study is part of a WABA accident-prevention campaign in cooperation with the D.C. Department of Transportation and other bicycling groups, which includes efforts to educate both bicyclists and motorists on safe driving techniques.

For more information on bicycle safety call WABA, 393-2555, or the D.C. Bike Office, 727-5824.