The area's transportation planners have been saying it for years: Washington's Metro is not going to eliminate the long-range traffic problems of the area, but it will make it easier to get downtown, at least for a few years.

Although only 17 miles of the planned 100-mile Metro rail system will be operating after July 1, that small segment will give area residents their first real clue as to just how much easier the trip will be.

The big winners will be those who commute by bus from Virginia. Their total travel times could be reduced by as much as half, depending on where they start.

Residents of Washington east of the Anacostia River and Prince George's County beyond will also have much improved public transit to downtown, although they will be poured through a very narrow funnel: the Stadium-Armory Metro station.

And those who continue to drive will prosper because Metro will remove a substantial number of bus trips from downtown streets, particularly from 14th, H and I Streets NW, and thus somewhat reduce traffic congestion.

The experience of other cities has been that the people who first make use of new public transit facilities are already transit customers. It takes time, good service and convenience to build patronage among those who are accustomed to driving their cars.

But an interesting phenomenon of Metro's rail experience with its first five miles - the link between Rhode Island Avenue and Dupont Circle - has been that the rush hour came at noon, not in the morning or evening.

From that, planners have inferred that many people are taking advantage of Metro's already good and rapidly improving downtown distribution network to shop or eat lunch in different neighborhoods from the ones in which they work.

Even that five-mile system, however, has attracted a solid core of 20,000 commuters daily - people who depend on the train to get to work and back. They usually use Metro in conjunction with their car or with a Metrobus.

Only one parking lot specifically located and priced to take commuters out of their cars and put them on the train is opening with the 12-mile section of Metro next Friday. That is at Stadium-Armory.

And although some auto drivers will doubtless find ways to use the Metro and park cheaply near other stations, the bus commuter is being encouraged, and in some cases forced, to transfer to the train.

After the initial irritation of dealing with something different - new fare structure, new fare-collecting system, new routes, new schedules - most bus commuters to downtown from Virginia and the eastern part of the metropolitan area should notice an improvement in the time it takes them to get to work.

Take, for example, a typical rush-hour bus trip from the Springfield Mall area of Virginia to Farragut Square NW. Today, that trip lasts 45 minutes, and more than half of that time is spent on the short leg within the District of Columbia.

The Shirley Highway express bus lanes, which have been highly successful in attracting commuters to buses - only extend to the north end of the 14th Street Bridge. There, the bus traveler is dumped onto Washington streets and his driver must cope with uncertain police enforcement of the curb bus lanes and intersection blockers. On bad days, it can take 30 minutes to go from the 14th Street Bridge to Farragut Square, a distance of just over 1 1/2 miles.

Metro will change that. After September 4, most of the Shirley express buses will go through the Pentagon, where the commuter can transfer easily to the subway. Travel time from the Pentagon to Farragut Square by subway will be six minutes, according to Metro.

Travelers from McLean and points west who take the bus today over Key Bridge are forced to creep through Georgetown to get to the Connecticut Avenue area. If they work on Capitol Hill, they have to go all the way across the core of downtown.

Beginning July 17, buses in that corridor will be rerouted through Rosslyn, where commuters can make a quick change to the Metro. Subway time from Rosslyn to Capitol South, which serves the House office buildings, will be 10 minutes. It will take 14 minutes to travel between Rosslyn and Union Station, which involves changing trains at Metro Center.

District of Columbia and Prince George's County bus commuters who normally come across the Anacostia on the 11th Street, East Capitol Street or Benning Road bridges will find a variety of bus-subway connections along the line that extends from Stadium-Armory to the federal employment core. Many bus routes will terminate at Stadium-Armory and, traffic planners hope, so will many automobiles, in the 1,200-place parking lot close to the station.

But this is just the beginning. The opening of the line from the Stadium to National Airport makes most of downtown Washington and major business areas of Arlington readily accessible by train.

The biggest potential impact on auto drivers won't come until later in the decade when Metro reaches the Beltway. If the presently planned, 100-mile Metro system is completed, nearly all of the residents in the District of Columbia, Arlington and Alexandria, along with those in large areas of Montgomery County, will be within 45 minutes by public transit to half of the region's jobs, according to a new study by the Metropolitan Washington Council of Governments.

Between 1985 and 1995, the COG report says, continued growth in regional job patterns will "offset any gains made in reducing commuting by auto to the central area and will result in large increases in travel within outlying counties."

In other words, the best Metro can do over the long run is keep the area even with its transit needs.