If there is no subway line form Gallery Place to Greenbelt, $766 million in construction costs will be saved but 21,300 more automobile trips will be taken every day in 1990.

If the subway line to Vienna is halted at Parkington in Arlington County and remaining service replaced with express bus service, $244 million in construction will be saved but Metro will have to buy 540 new buses that will annually consume 10 million gallons of fuel more than the subway would.

Those kinds of choices are spelled out in a new, 5 1/2 inch thick that was generated to help area politicians decide whether buses, trolleys or more freeway lanes might be better than subways for some parts of the region.

The report, from the accounting firm of Peat, Marwick, Mitchell and Co., will provide the groundwork for decisions that will affect transportation in the region for decades to come.

At the same time, there is growing concern among transportation planners that the decisions will be made with more attention paid to intial, out-of-pocket cost than to long-term public transportation needs. That is because the politically sensitive property tax is the only means area jurisdictions have found so far 10 finance transit.

The PMM report discusses numerous alternative transportation systems for four lines on the planned 100 mile Metro system: The Greenbelt line, from the existing Gallery Place station at 7th and G Streets NW, through Northwest and Northeast Washington, College Park and Greenbelt.

The Branch Avenue line, connecting Southwest Washington with Anacostia and Prince George's County.

The Springfield line, connecting the planned King Street station in Alexandra with the Springfield-Franconia area of Fairfax County, near Springfield Mall.

The Vienna line, connecting the cross-Arlington route with Falls Church and western Fairfax County's Interstate 66 corridor.

Sixty miles of Metro's planned system are either under construction or have guaranteed funding.A few miles, including the controversial Glenmont line from Silver Spring, are the subject of a separate study now under way at Metro and the U.S. Department of Transportation.

The rest are studied in the "alternatives analysis" document that PMM began handing out to area officials last week. The document presents no recommendations or conclusions, but does provide the kinds of analytical information that can form the basis for decisions.

The study was ordered by the federal government a year ago when the growing costs of Metro construction - combined with rising operating deficits and uncertain local commitments to meet them - caused financial planners to take a second look.

Metro construction had been proceeding merrily along since 1969. During that time, a period of enormous inflation, the estimated cost off of completing the 100 miles rose from $2.5 billion to more than $3 billion. Under the present formula, the federal government would provide 80 per cent of the construction costs.

The schedule now calls for the regional task force to take the PMM study and choose three alternatives from each transportation corridor. The study offers five or six choices for each.

Then, the three alternatives for each corridor will be boiled down into three regional systems - a truncated Vienna line but a completed line to Branch Avenue for example - and compared with the original 100 mile system.

The federal government also wants a long range financial plan that would be, in effect, a commitment to run what is built. Regional politicians are beginning to study various types of local taxes that could be used to take the transit cost off the property tax.

In addition to the obvious comparisons of construction costs for lines truncated at various places, PMM has attempted to provide other measure of transportation effectiveness with each alternative. Included are such measures as the size of the operating deficit and the impacts on transit ridership, car usage and energy consumption.

The various proposals for each corridor, briefly summarized, are as follows: Greenbelt Line

Alternatives included a completely realigned rail system in Prince George's County with a terminal at the junction of Interstate Rte. 95 and the Beltway instead of at Greenbelt.

Other proposals include truncations of the Greenbelt line at Prince George's Plaza. Fort Totten (where the Greenbelt line would intersect the soon to-open Red Line extension, Columbia Heights (at 14the an Kenyon Streets Nw) and at Gallery Place. That last proposal includes two new trolley lines in the corridor and some improvement to the B&O commuter rail service between Laurel and Union Station. The consultants also studied a "do-nothing" alternative.

Branch Avenue Line

The present route follows Suitland Parkway to a point near the Beltway and Branch Avenue. One alternative would realign that route generally along Wheeler Road in Southeast Washington to a terminus near the Rosecroft Raceway in Prince George's County. Other alternatives include a combination of the two routes, and truncations at Southern Avenue and Wheeler Road or at Minnesota Avenue and Good Hope Road SE, the planned Anacostia station. Improved bus service accompanies some alternatives. Springfield Line

Alternatives include not building the line at all, or truncating it at either the Van Dorn or Springfield stations. Another alternative is commuter rail service along existing tracks of the RF&P and Southern railroads. Vienna Line

Alternatives include truncating the line at Ballston, near Parkington in Arlington, or at West Falls Church. Also discussed are a new alignment along the Dulles Access Road to Tysons Corner or Dulles Airport. Direct bus service from various points in Fairfax County to the Rosslyn and Pentagon stations is also discussed.

It is impossible in a short summary to explain the infinite variations that are conceivable for the metropolitan area. Each of the alternatives also includes various transit ridership projections based not on the length of the subway line alone, but also on the availability of parking at the various subway stations.

The federal government, through the Department of Transportation's Urban Mass Transportation Administration (UMTA) is requiring studies similar to the one here for any city seeking to make a major transit expenditure.

Federal funds are limited, is what officials have been hearing, and everybody has to compete with everybody else for the transit dollar. Therefore, UMTA officials have warned, they must be certain that the system that localities propose must also be the most cost-effective one.

Transportation Secretary Brock Adams and UMTA chief Richard S. Page have placed particular emphasis on the need for local land use policies to encourage high density construction around subway stations. If needed densities are lacking or cannot be easily brought to the station (in a giant parking lot), they have been saying, then an expensive subway line may not be justified.