Washington-bound commuters have been abandoning buses and car pools, preferring instead to drive alone in their cars or switching to a new transportation tool, van pools, according to a study released yesterday by the Metropolitan Washington Council of Governments.

The number of van pool commuters heading into the city from outside the beltway jumped 75 percent between the fall of 1980 and the fall of 1981, and was up 260 percent from 1978, the COG biennial beltway "cordon count" found. However, van-poolers still represented only a relatively small fraction of that commuting population, 6,000 persons out of 357,000.

The unusual growth in van-pooling surprised area officials, especially since car-pooling dropped by 8 percent or 10,000 riders during the same study period.

The majority of suburban commuters crossing the beltway, 205,000 or 58 percent, drove to work by themselves in their cars, the study found. That represented an increase of 8,000 single-occupant vechicles, or a 4 percent rise, even as the total number of commuters crossing the beltway remained roughly the same, the report stated.

The increase in single-occupant commuting came as no surprise to Metro or other local officials, since "we're offering the public less service at a higher cost" than in 1980, said Robert Pickett, Metro's assistant director of planning and development. The number of transit riders fell by 3,000, or 10 percent, during the period. Metro has had several fare increases since 1980 and has cut numerous bus routes.

In separate actions, COG's transportation planning board yesterday was told of two major car and van pool improvements, a much faster and better Commuter Club computer matching service for area residents and a feasibility study that recommends 12 miles of additional bus and car pool lanes on Shirley Highway, as far south as Dale City.

Virginia highway officials in 1980 recommended a 19-mile extension of the bus and car pool lanes from Springfield, where they now end, to the Quantico Marine Base. The COG study says the shorter distance, estimated to cost $77 million to build, would be sufficient for at least the next decade. The project is several years off in any case, since Virginia has no money for it, the COG board was told.

Improvements in the computerized matching of would-be car- and van-poolers mean that program now can provide names, home and office addresses and working hours of more than 30,000 commuters, and mail out the information in a week or less. Previously, it took as much as a month.