The United States is experiencing a motor vehicle population explosion, a Brookings Institution researcher has found. The boom has reached the point where there is more than one vehicle for every two people, and, in absolute numbers, the vehicle population is growing 2.5 times as fast as the human population. What's more, according to Anthony Downs, a senior fellow at Brookings, the growth has continued undeterred by the energy crisis of the 1970s. In percentage terms the sharpest growth, he adds, has been in trucks, which have increased about twice as fast as cars. The explanation, says Downs, is "a rapid rise in multicar ownership." In 1955, only a tenth of all households owned more than one car. But by 1974, a third of them owned two or more. "In fact, when trucks are taken into account, only 18.5 percent of all households did not own any vehicles in 1974 and 45.1 percent owned two or more." And the future? Still more cars and trucks, he predicts, perhaps as many as 320 million by the end of the century. Gasoline has continued to be a relative bargain, he contends; when inflation is taken into account, a gallon of gas was cheaper last year than it was in 1950, climbing above the 1950 level only during the recent price surge. "If, however, serious absolute [gasoline] shortages develop and remain in existence for long periods, total miles driven will have to be cut back drastically," he says, but he believes "this would be accomplished through more use of carpooling, vanpools and careful trip planning, rather than by any massive switch to public transit. . . . In that case, total vehicle population will increase more slowly."