DEEP IN THE TUNNEL just south of the Pentagon Metro station, the concrete wall is interrupted by a Y-shaped cutout to accommodate a junction for a new subway line. It will run southwest from the Pentagon under Columbia Pike, out through Bailey's Crossroads to Annandale. If you haven't heard of Metro's Columbia Pike Line, don't feel left out. It will never be built.

Neither will the Outer Beltway, another loop around the suburbs; the Potomac Freeway, an expressway along Canal Road; the Inner Loop Freeway, a mini-beltway around downtown Washington; the North Central Freeway, to carry Interstate 95 under the Mall and north through the middle of Washington; or the Three Sisters Bridge, another Potomoc River crossing north of Key Bridge.

All these projects were once seriously regarded by planners or developers or politicians (or all three) as musts. All have been abandoned.

While the story of Washington area transportation over the past two decades includes many minor improvements and at least two remarkable achievements -- the Capital Beltway and the Metro subway -- it is also full of mistakes and missed opportunites. The challenge of the next decade is to see if some of those mistakes can be corrected and their repetition avoided as the next round of development pushes the metropolitan area outward.

Albert A. Grant is the quiet, thoughtful director of transportation planning for the Metropolitan Washington Council of Governments and a former highway engineer in the District of Columbia. He best stated the themes of this article when he said in an interview, "There is no question that Metro has been a plus. The only minus is that, because of naive hopes that Metro could serve all demands in the region, highway construction was deferred."

THE CUTOUT in the wall of the Pentagon tunnel is but one example of how the area's politicians have a genius for solving political problems while sidestepping the underlying issues.

In 1968 the politicos met in the Statler-Hilton Hotel (now the Capital Hilton) to make the final decisions about Metro's pathways. Those from Maryland -- primarily those from Montgomery County -- did not want Virginians to have three subway lines, and certainly not the Columbia Pike line running through what was then the heart of developed Fairfax County, because the high cost of that construction would reduce the money available for the rest of the system. Maryland found an ally in Alexandria Mayor Charles E. Beatley, who figured, he said later, "if the Columbia Pike line was never built, there would never be a Skyline Center."

Beatley was opposed to the enormous Skyline development, slated for construction just outside Alexandria's border, because it would flood the city with new traffic while offering it no offsetting new tax revenues.

The Columbia Pike line was stricken from the Metro system. The Y-shaped cutout was designed into the Pentagon tunnel to make it easier to add the line if Metro got around to extensions beyond its planned 101-mile system, which itself now may never be completed.

As for Skyline Center, there it is nonetheless, a massive high-rise complex at Bailey's Crossroads, next to the Alexandria city limits. The traffic jams on nearby Rte. 7, particularly on the permanently unwidened section through Alexandria to Shirley Highway, are just short of legendary. So much for stopping development by not providing needed transportation.

The area is full of such examples, the most egregious being Tysons Corner and the Springfield Mall-Backlick Road areas in Fairfax County and the White Flint-Rockville Pike mess in Montgomery County.

Other problems are developing, particularly in western Fairfax County and in northern and eastern Montgomery County; and those are problems that Metro may never be able to help. Prince George's County is on the verge of similar difficulties, particularly to the east toward Annapolis and to the south toward Charles and Calvert counties.

R. Robert Linowes, a lawyer specializing in real estate development who started in Montgomery County and now has offices downtown, is a bit of a cynic about how these things happen. He said, "We tried to create a philosophy that says, 'Don't do anything and everything will go away' . . . It's going to take a massive infusion of guts and backbone" to correct.

"The politicians," he said, "avoid the problem by playing games about where roads should be located. In suburban Maryland generally and Montgomery County specifically, they study, they create task forces, they hold hearings, but they never make the hard decisions. So they can tell the economic development bunch that, 'We're trying to resolve it,' and tell the other side, 'There's no way it will be built.'

As for the District of Columbia, Arlington and Alexandria, they have Metro -- the result of a conscious decision to put their dollars in the subway instead of in freeways.

The District of Columbia chose not to build 17.2 miles of interstate highways. The unbuilt roads would have ringed downtown with high-speed lanes, would have put an expressway through West Potomac Park (next to the Lincoln Memorial), and would have put a freeway through Northwest Washington with branches to the Beltway extending through both Montgomery and Prince George's counties. There would have been yet another freeway to accompany the Whitehurst along the Georgetown waterfront; there would have been the Three Sisters Bridge across the Potomac, and another freeway through or under the National Arboretum and along the west bank of the Anacostia River.

Those freeways were stopped by angry citizens -- often led by recently defeated Takoma Park Mayor Sammie Abbott, a pugnacious, confrontational white man who fought the roads with the devastatingly accurate slogan: "White men's roads through black men's bedrooms."

Few today think the decision by Washingtonians to abandon freeways was wrong. City administrator Thomas M. Downs, who worked in various U.S. Department of Transportation jobs before joining the District government, said, "If you think about what would have happened if Interstate 95 had come into town, we would be debating the need for two or three more lanes on the Southeast-Southwest Freeway, we'd have trucks overturning, everything else. I think it was a pretty good decision."

So $2.6 billion in freeway money was spent on the subway and 1,000 to 1,200 homes and apartments were saved. The result is a system that wins wide approval in every local poll, a system that tourists love, a work of political genius placed in the ground by a no-nonsense retired Corps of Engineers general, the late Jackson Graham. But it was, and is, expensive. The original estimate for the entire system was $2.5 billion. That was in 1968, before the extraordinary inflation of the 1970s. Metro's official projection now is $9.4 billion but some estimates run as high as $12

The subway opened 10 years ago (its anniversary is March 29 as the Red Line stretch from Rhode Island Avenue to Farragut North. The next segment to go into operation, on July 4, 1977, was a portion of the Blue Line, connecting the House of Representatives to the Pentagon and National Airport, and carrying the representatives of the military-industrial complex to RFK Stadium to watch the Redkins. The political base to guarantee Metro's future was there in its second year.

While the political genius of the construction schedule cannot be denied, one consequence was that the one Metro line that would serve the greatest number of so-called "transit dependents" was the one saved for last: The Green Line is supposed to run from southwestern Prince George's County through Anacostia and Southwest Washington, up the

Seventh Street and 14th Street corridors that were devastated by the 1968 riots, then northeast into tired but regenerating Prince George's County neighborhoods in Hyattsville, College Park and Greenbelt.

That sections of the Green Line are just now being built to Anacostia is not so much a matter of ill will as a matter of not noticing. Montgomery County and Washington west of Rock Creek Park have always been able to take care of themselves; the lawsuits that had to be pursued to establish Metro rights of way were pursued, the political battles were fought. In Prince George's County and in Washington east of the park, there was less political will or skill.

JAMES S. MINOR has been with D.C. Transit and Metro as a bus driver for 17 years. Now, at 6 a.m. -- after 12 weeks of training, one week of practice in the yard and four days on the line -- he is taking Train 309 from the Alexandria yard to service on the Yellow Line. He is just a bit nervous.

"I don't know how this happened," Minor said. "That man's in the cab and he knows everything about how this works." "That man" is Fady P. Bassily, the Metro assistant general manager in charge of running the railroad.

For a reporter familiar with the sometimes careless and procedureless operation that was the Metro subway when it started running, this morning on the line 10 years later is affirmation that Metro has grown up; but it took a 1982 accident that killed three passengers.

Breakdowns occur less often now and the subway recovers more quickly. The car doors open and close instead of sticking. The sometimes unreliable destination signs are being replaced. The stations now have names on the outside, where they can be seen.

As he provides his Cook's tour of the system, Bassily fills three cards with notes on things to fix. His temperament, he concedes, has sometimes worked against him. "I have a low tolerance for incompetence," he said. "If the Lord wanted to create us dumb, He wouldn't have created us in His own image."

If you take away Metro, the city strangles. More than one-third of the people who come to downtown Washington between 6:30 a.m. and 9:30 a.m. daily do so on transit. Areawide, those who use Metro's buses and trains make about 718,000 trips a day. But Metro is not cheap. The major long-term challenge is a reliable subsidy for the combined bus and rail system, which spends about $2 for every $1 it collects in fares. About half of Metro's $423 million budget comes from state and local taxes.

"I think there should be a regional tax, not only for Metro but for transportation generally," said Linowes. "From a practical, realistic point of view, it's the real solution if we are willing to face the problem."

Suburban jurisdictions are pulling out of the Metrobus feeder system to start their own bus networks, meaning better bus service for some commuters at the cost of a subtle threat to the regional alliance. The District government is afraid it will be left with Metrobus, its costs and labor contracts, while the suburbs cut and run.

WHEN METRO'S Vienna line opens next summer, 69.6 miles of the planned 103-mile rail system will be in place. Four spokes from downtown will reach out to the Beltway to intercept commuters.

For hundreds of thousands of other commuters, however, Metro is useless. It has no meaning for someone going from Oxon Hill to Fairfax or from Vienna to Bethesda. They go by Beltway, the 66- mile ring road that was completed in August 1964, when The Washington Star predicted, "In the years to come, suburban development will center more and more around the major exits." That quali- fies as prime understatement in transportation literature.

The Beltway was part of President Dwight D. Eisenhower's 42,500-mile interstate highway system, which changed the face of the nation and its cities, providing easy access to suburbs and opportunities for the real estate business.

The Beltway was designed to be a bypass, to divert interstate traffic from the core. Instead, it has become Washington's rush-hour Main Street, carrying one mile of every 10 driven in the area. Exit lanes at major interchanges cannot begin to handle the evening rush hour.

"One major failure was that when we built the Beltway, we recognized we would need another one," Linowes said. "We recognized it and ignored it. That has exacerbated the problem we're having now. The politicians were willing to put it on a long- range plan, but unwilling to carry it out." The plans were for an outer beltway looping five to eight miles beyond the present Beltway. It has long since been dropped from the maps, but two of its most important segments are returning: the Springfield Bypass in Fairfax County, which will carry autos from Shirley Highway to Rte. 7 west and south of the present Beltway, and the Intercounty Connector in Maryland, which will someday run east from roughly the Shady Grove Exit on I-270 to Rte. 1 in Laurel.

Just as the 1970s was a decade of major anti-freeway, pro-transit sentiment in the Washington area, the 1980s and 1990s may well bring the return of the road builders. One of the reasons is John T. (Til) Hazel Jr., past president of the Greater Washington Board of Trade, a lawyer who made his reputation winning rezoning cases in Fairfax County and who says now, "I spend most of my time in development."

"All of a sudden, in the last four or five years, guess what?" he said. "There are no roads, there are no plans, Metrorail is open and very expensive and the other guy isn't riding it enough. All of a sudden (the voter) goes to the politicians and says, 'Fix it, or else.'

He thinks "there ought to be both a western and an eastern bypass" of the Washington area. The western bypass, outlined by the Virginia Department of Highways and Transportation, would start at I-95 north of Fredericksburg, cut west of Washington between Dulles International Airport and Leesburg, cross the Potomac and western Montgomery County, intersect I-270 and tie in with I-70 and Rte. 40 near Mount Airy.

The eastern bypass would run through Maryland somewhere along the Rte. 301 corridor. It would be easier to build these new roads rather than to perform corrective surgery on existing highways; and they could be paid for with tolls and levies on developers.

"I think you have to be very careful about where you put interchanges and where you give local development opportunities," Hazel said. "If upper Montgomery County doesn't want" to permit local access, that's Montgomery County's decision to make, he sid. In Virginia, "We just need access at Routes 29, 50 and 7."

When Hazel was president of the Board of Trade in 1984, he arranged for a public opinion survey. The poll revealed that suburbanites believe they do not have adequate roads and are willing to pay for improvements. The accuracy of the finding has been demonstrated in Fairfax County, where residents overwhelmingly approved a $135 million road-building bond issue in the last election, the third time in recent years they have voted to tax themselves for roads.

But no matter where the money comes from, commuters are apparently going to demand their roads be built.

"The only thing we're going to be able to do is catch up to where we should have been five to 10 years ago," Linowes said. "The big concern is to avoid being 20 years behind."