Northern Virginia commuters, especially the area's 60,000 long-distance commuters, should expect significantly worse traffic jams and slower travel times during the next 10 years, a new state commuter study has concluded.

The study, prepared by the traffic consulting firm of Barton-Aschman Associates for the Virginia General Assembly urges the legislature to encourage ridesharing -- carpools, vanpools and privately operated buses -- as the only feasible and cost-effective way of combatting rush-hour congestion in the region. The study concludes that unless gasoline prices go up noticeably, ridesharing is unlikely to increase much or have much impact on traffic congestion, even with strong government support.

It recommends numerous programs to aid ridesharing, to support private car and vanpools, which it estimates would cost about $2 million a year, compared to the $32 million the state will spend this year in support of mass transit and ridesharing.

The study found that mass transit, commuter trains and buses are useful only in close-in suburbs of large urban areas and on certain heavily traveled corridors such as Shirley Highway. It recommends against increased future support for expanding mass transit beyond present areas, especially since federal operating funds for transit are being reduced by the Reagan administration.

Because Washington's suburbs are growing faster than the downtown business district, the study predicts "major changes in commuter travel patterns" in Northern Virginia and increased traffic congestion at Tysons Corner, Springfield, and other major suburban employment centers.

The study says that for most long-distance commuters -- those who travel more than 10 miles to work--ridesharing is the only cost-effective alternative to driving, because the commuters live in dispersed areas and commute to increasingly dispersed employment centers.

It cites the increasing use of carpool lanes on Shirley Highway and the opening of I-66 where only carpools will be permitted during rush hours as good examples of public support for private ridesharing.