IT WAS a marvelous opportunity to make a point. There was Rep. Dan Rostenkowski (D-Ill.), sitting in a limousine with President Carter as they sped toward downtown Chicago from O'Hare International Airport. There, on the other side of the freeway, was a five-star, four-lane rush-hour traffic jam.

Rostenkowski, as he recalls it, turned to Carter and said something like this: "Mr. President, we will greatly avoid this congestion once the O'Hare extension [to the Chicago subway-elevated] is completed. Not only will it be a matter of convenience, but look at the energy those cars are using just sitting there."

There it was, the energy argument for transit: If people ride transit they aren't driving; if they aren't driving they aren't using gasoline; therefore, transit saves energy.

Does it really? Does transit save enough energy to justify a $15 billion program over the next decade, as Carter has proposed and as Congress seems likely to approve?

The answer is yes, but not for the reasons commonly argued. Because such a small percentage of the American public can use transit -- even if there are major increases in ridership -- most gains in energy consumption will have to come from elsewhere.

However, major transportation investments -- such as a subway line or an exclusive freeway lane for express busess -- have a demonstrated ability to encourage high-density, energy-efficient development. Given a period of years, the patterns of total energy consumption -- not just transportation energy -- can be changed for good or for ill by major transportation investments.

Finally, as Rostenkowski was pointing out to Carter, there are congestion and urban livability problems that transit can ease. When a city as traditionally auto-dependent as Houston votes to tax itself for a transit system, as it did last year, there is a strong indictation that automobile traffic has reached the saturation point.

Former Transportation Secretary Brock Adams put it this way: "I don't care if you find a car that uses no energy and creates no pollution. If everybody drives by himself downtown, there isn't going to be room enough to park."

The goal of the Carter administration's transit proposal -- part of the windfall profits tax package now working its way through Congress -- is to add 15 million passengers per day to the 28 million who already ride the nation's transit systems.

That sounds like a big increase, but it is really only 7.5 million people since everyone is presumed to make one round trip (thus two trips) per day.

The estimated savings in oil consumption if those 7.5 million people start using transit is 115,000 barrels a day, according to the Department of Transportation. That works out to 4.8 million gallons of gasoline a day, and sounds like a lot of gasoline.

However, during the first eight months of this year, according to the Department of Energy, gasoline was consumed at the rate of 7.1 million barrels -- or 298.2 million gallons -- per day.

Therefore, for $15 billion, the Carter administration is proposing to reduce gasoline consumption by 1.6 percent. Furthermore, the number of people who actually use transit for the home-to-work trip will still total less than 5 percent nationally, although the percentage exceeds 50 in some northeastern cities. Most of the rest of those trips will be made by automobile.

Clearly, the big reductions in national gasoline consumption are going to have to come from such things as more efficient automobiles, more restrained use of those automobiles and increased car pooling.

A watershed in that sort of computation was reached in September 1977 when the Congressional Budget Office produced a study that attacked new heavy rail transit systems, such as Metro and San Francisco's Bay Area Rapid Transit (BART) because, the study said, such systems "acutally waste energy rather than save it."

"When the factors as construction energy, the energy used to get to and from stations, and thte roundaboutness rail travel involved are considered, the energy per passenger-mile computed from door to door for rail rapid transit is greater than that for any other public mode except dial-a-ride," the study said.

That study succeeded in uniting the transit lobby in the United States in a way that had never happened before. Big city transit systems from across the country buried Congress in technical rebuttals to the analytical process used by the CBO and in factual corrections to some of the CBO's assertions.

The fact that rail transit systems can run on readily available coal rather than petroleum was heavily underlined, as was the contention that the very existence of a decent transit system reduces the amount of gasoline purchased, if not the number of automobiles owned.

Milton Pikarsky, former chairman of the Regional Transportation Authority in Chicago and now director of transportation research for IIT Research Insitute, pored over gaoline sales statistics and produced a table showing the average gasoline purchase per week per licensed driver in the following cities: (TABLE) 02,00,14 Manhattan(COLUMN)4.43 gallons Chicago(COLUMN)6.84 New York City (all boroughs)(COLUMN)9.76 Washington(COLUMN)8.19 San Francisco(COLUMN)12.17 Los Angeles(COLUMN)13.97 Houston(COLUMN)16.17 Bridgeport, Conn.(COLUMN)17.84(END TABLE)

The major problem with the CBO study and with the kinds of figures presented earlier in this article is that they look at transportation costs and efficiencies through a very narrow window. In fact, CBO recognized that in its report when it said, "It is assumed in the analysis . . . that homes, jobs and business do not move because of the changes in transportation programs . . ."

Well, homes, jobs and businesses do move because of changes in transportation programs.The evidence is available in the Washington area:

The Capital Beltway was completed in 1964, and in the next decade Washington moved to the suburbs. Now, however, with a third of Washington's subway system in operation, developers are fighting over downtown parcels of land they wouldn't touch two years ago.

The Federal City Council released a study last July that showed that more than $970 million worth of private development has taken place near Metro subway stations and another $5 billion is contemplated if the entire 101-mile system is completed.

A recent Urban Land Institute (ULI) survey found major new downtown office development in Philadelphia, Pittsburgh, Denver, Detroit, Milwaukee, Columbus and Chicago, among other places. All of those cities also have major transit improvements either under way or in the final planning stages.

J. Thomas Black of ULI said that "we are exploring what downtown office growth means for transit. It is clear they are highly interrelated, because this kind of [office] growth cannot be auto-related."

The emphasis of the transit plans for new downtown office developments has been to improve mobility in congested areas, not necessarily to save energy. Energy savings result, however, not only because people are encouraged to avoid driving downtown but also because it is known that people will choose to live closer to major transportation facilities that make it easy for them to get to work.

In the Washington area, for example, there is enormous private developer interest in building dense residential sections around outlying Metro stations.According to the Federal City Council survey, 2,900 private market dwelling units have been constructed close to Metro stations since 1976; another 9,200 are firmly committed or under construction and another 28,750 are regarded as "potential." Most of those units will be apartments or townhouses.

Another already visible example is the string of highrise condominiums that grew up around the Shirley Highway corridor in Northern Virginia, where excellent express bus service was established early in this decade. Before Metro's subway connection across the Potomac River opened in 1977, half of the people who crossed the 14th Street bridge from Virginia into Washington were doing so on the bus.

Row houses and apartment buildings share walls and need less heat or cooling than stand-alone ramblers. Under proposed Department of Energy standards, a new gas-heated single-family row house in the District of Columbia should use no more than 23,000 BTUs (British Thermal Units) per square foot per year for heating and cooling.

A detached home, however, would be permitted to use 31,000 BTUs, a third more. The consumption permitted differs from city to city across the country, depending on weather and climate, but the energy-saving relationship of shared walls remains about the same.

It is obvious from experience here and elsewhere that the success of a major transit investment such as Metro -- and thus its ability to effect energy savings -- will depend on land use that surrounds the stations. Not only do higher density communities or business centers close to Metro stations generate more passengers, they also require less energy than single-family subdivisions or shopping malls.

Despite that, when a new subway system like BART or Metro is inserted into an established area, political pressures to avoid "Manhattanization," especially around suburban stations, become enormous.

Jane Rogers, a land-use and growth expert with the Metropolitan Washington Council of Governments, notes that "you can see the schizophrenia" of local politicians as they attempt to satisfy the competing pressures against high-density development and the need to recapture their enormous expenditure for Metro through higher tax income generated by (incidentally) energy-efficient, dense development.

As Metro grows outward, those debates become quite heated. Alexandria is in the throes of deciding how high it should permit buildings to go around its King Street station. The City Council has proposed a height limit of 77 feet; developers have proposed 110-foot buildings, and present zoning permits 150 feet. Residents of the politically influential neighborhood west of the station are opposing the larger development while saying they can't wait for Metro to arrive and speed them downtown. There are similar examples throughout the region.

The bigger picture -- the one in which transit is seen as having a ability to conserve energy beyond that which would otherwise be used to travel -- appears to have been a key element in the growing political acceptance transit expenditures have received in recent months.

Another obvious factor is the surge to overloaded transit systems that occurred last summer as part of Energy Crisis II. Ridership nationally in July of this year was 36 percent higher than in July 1972, according to the American Public Transit Association. Readership on transit systems around the county has now increased every year since 1973, when it hit bottom after a steady decline dating to the end of World War II.