Thousands of Northwest residents will face a double whammy -- longer commutes to work and higher fares -- if Metro cuts bus service after three new subway stations open in December.

District transportation officials are proposing cutting back or eliminating all L route buses along the Connecticut Avenue corridor, to force more than 4,000 riders onto subways and save the financially struggling transit system an estimated $460,000 a year in bus operating costs. The proposals also would affect Montgomery County riders.

Under the plan, which would start in January, Connecticut Avenue Metrobus routes would terminate at one of the three new Red Line subway stations planned to open Dec. 5: Van Ness-UDC, Cleveland Park and Woodley Park-Zoo. Later in 1982, all Connecticut Avenue L buses would end at the Van Ness station.

The proposed Metrobus changes will be discussed at public hearings Sept. 8 at the county office building in Rockville and Sept. 9 at Lafayette Elementary School, Broad Branch and Northampton roads NW. Both hearings begin at 7:30 p.m.

The proposals by District officials could be modified by the Washington Metropolitan Area Transit Authority (WMATA) after the public hearings. Neither the WMATA staff nor Montgomery County officials have yet taken a position on the proposed changes.

For most current L bus riders, the changes will mean a transfer to the subway, instead of a single bus ride into downtown Washington. Some riders to places like the State Department, Department of Interior and Federal Triangle may have to take two buses and the subway or three buses in order to reach destinations they now get to with one direct bus ride.

"That's a double whammy that may lose us some riders," said Metro planner Richard Dawson, "but we may pick up new riders who now drive . . . and these proposals (stopping all the Connecticut Avenue buses at the subway stations) are not final, they're just proposals."

A recent study by Montgomery County transportation officials estimates that the proposed changes on the nine Connecticut Avenue L routes will raise the cost of commuting to downtown Washington (Farragut Square) about 25 percent and add 5-8 minutes to the commuting time for most riders.

For riders who travel beyond Farragut Square, the plan would boost fares 30-35 percent and increase commuting times 5-10 minutes because further transfers would be needed, according to Edward Daniel, chief of the county's transit services division.

For some commuters along the Connecticut Avenue corridor, traveling time would decrease in 1983 when the Red Line is scheduled to open stations all the way to Shady Grove Road near Gaithersburg. Then, many more Northwest and Montgomery County riders will be able to ride the subway without connecting buses and extra bus fares.

Commuters interviewed on L buses last week predicted their trips would increase much more than 5-10 minutes, as well as cost more, although Metro planner Dawson predicted that transferring to the subway will save many commuters "as much as 5 minutes."

"It will cost me a lot more money and a lot more trouble," said Frank Stearns, an Interior Department employe who rides the full L5 route from his home near Chevy Chase Circle to Potomac Park by the Interior building at 18th and C streets NW. He will have to bus to Van Ness, take the subway to Farragut and either walk or transfer to another bus.

One State Department employe, John Holzman, said, "I swear to God I'll find another way of going," rather than taking two buses and the subway to get from Chevy Chase Circle to work. Holzman regularly jogs the five miles home. His neighbor and fellow State employe, Herbert Thomas, who frequently bicycles to work, also has other options.

Apparently many commuters go out of their way to use the L buses because Connecticut Avenue is considered a fast route. "I come from Bethesda and could go down Wisconsin Avenue," said another State employe and L5 rider, Martha Sardinas, "but it will really be great when the subway reaches Bethesda."

Forcing riders to transfer to the subway and then perhaps another bus, "will mean a lot of wear and tear on everybody, especially elderly people," said Eileen Binns of Potomac, whose husband drives her to Connecticut Avenue. "It's hard to convince people like my husband to ride public transit if it's difficult," she said.

When coupled with the system-wide fare increase proposed by Metro for this fall, the cost of traveling by Metro along Connecticut Avenue could rise as much as 35-40 percent. How much of the proposed 14 percent increase suburban riders would pay has not been determined. Metro will hold a series of seven public hearings on the general fare increase between Sept. 16 and Sept. 30.

If the L bus route changes are approved after the Sept. 8-9 hearings, they would go into effect in January. Higher bus and subway fares of some kind will begin Dec. 5, with the opening of the new Red Line stations.

Some previously approved changes in bus routes will begin Sept. 6, with others delayed until January. They were among a series of controversial bus cutbacks on 44 District and Maryland bus routesdiscussed at public hearings in May. The most controversial proposals, eliminating popular D1, D3, N1, N3 and P1 bus routes, have been indefinitely postponed by the regional Metro board.

One popular bus route, the L5, was marked for elimination in May, then apparently saved after strong public opposition at the hearings. It is again slated for extinction as part of the changes on all L routes. Most of the nine daily L5 buses have standing room only.

The most crowded bus route, L7, which carries about half the Connecticut Avenue bus riders, also would be eliminated. But most of its riders get off at Farragut Square, right by the subway stop, said Dawson, so riding the subway will not be a great a hardship for them.