In a cost-cutting effort, the Metro transit authority proposed yesterday to halt service next year on 18 bus routes in Alexandria and Arlington County--a move expected to cause inconvenience for many Northern Virginia commuters.

The proposed cutbacks are expected to go into effect several months after rail service on Metro's Yellow Line is extended to Alexandria and Fairfax County. Hundreds--perhaps thousands--of commuters who now ride buses would be forced to transfer to the rail system at the new stations, according to transit officials. Some passengers would face longer trips and higher fares.

According to recent estimates, which officials said would likely be revised, Alexandria would save $1.7 million a year in Metro costs under the transit authority's proposal. Arlington's savings were forecast as about $400,000 annually.

The Yellow Line is tentatively scheduled to be expanded in December to include three stations in Alexandria and a terminus near Huntington Avenue in Fairfax County. Metro's schedule was recently thrown into doubt, however, because of delays in testing new rail cars, which are needed to open the four-mile extension.

Nevertheless, optimism is now mounting among transit officials that the spur from National Airport to Huntington may open on time. One scheme under consideration is to start operating the extension with shorter trains than previously planned, meaning that fewer new cars would be needed.

Overcrowding might be avoided on the shorter trains, officials have said, if the proposed cutbacks in bus service were delayed--fewer bus riders would then be forced to switch to the rail line.

The transit authority initially planned to reduce bus service in January, but is now considering making the changes between March and June.

Proposals by Metro officials for additional cuts in bus service in Fairfax County were recently rejected by Fairfax officials.

Fairfax Supervisor Joseph Alexander, a member of Metro's board of directors, contended that the plans would have led to higher fares and longer trips for commuters and would not have saved the county any money.

Indeed, Metro's estimates showed that Fairfax County's annual expenses would increase by $700,000 despite the suggested cutbacks in bus service. A new study of possible changes in Fairfax bus routes is under way.

The transit authority normally reduces bus service when it opens new rail stations. Its aim is to avoid excessive costs stemming from overlapping bus and subway service. Transit officials argue that, although some bus riders are inconvenienced, other commuters benefit from faster rides and the transit system attracts new passengers who previously traveled by car.

The Alexandria and Arlington bus routes which Metro officials recommended eliminating are the 6B, 6E, 6F, 11E, 11G, 11X, 12A, 12E, 14A, 14B, 15A, 15B, 15D, 19G, 19Y, 21C, 25S and 28A.

In addition, changes in routings or schedules were proposed on many other bus routes, and two new routes were suggested.

Metro's board scheduled public hearings on the proposals for Oct. 26 in Alexandria and Nov. 2 in Arlington.

The board may alter the plans as a result of testimony at the hearings or recommendations by local officials

In another development, the transit authority issued a report showing significant improvements in safety since the mid-1970s.

It said bus accidents decreased 47 percent since 1974, injuries to bus passengers dropped 14 percent, construction accidents declined 80 percent, injuries to subway riders fell 94 percent since 1976 and accidents in rail stations decreased 75 percent.