Soaring gasoline prices are making many Americans cut back on their driving, and highway travel is "rapidly reverting to 1977 levels," the U.S. Department of Transportation has reported.

The agency said highway travel during March dropped to 6.1 percent less than in March a year ago, when the slowdown began.

Since then, the number of miles driven on U.S. highways has declined steadily, the DOT reported.

The 6.1 percent drop in March driving was the second sharpest year-to-year falloff ever, only slightly less than the 6.3 percent decline registered last July in the midst of gasoline shortages.

The decline in driving now despite abundant supplies of gasoline tends to "support the contention that gas prices are a major factor in energy conservation," said the DOT Federal Highway Administration.

The Transportation Department pointed out that the 120 billion vehicle miles traveled on U.S. roads during March was less than in March of either 1979 or 1980 and just fractionally above the March 1977 level of 119.9 billion vehicle miles.

The agency said the trend in travel appears to be a decline to the level of three years ago, effectively wiping out two years of growth in driving.

The DOT didn't release figures on gasoline consumption, but the trend toward smaller cars apparently means Americans not only are driving less, but also using less gas for the driving they do.

For many years highway travel in the United States had been increasing by about 5 percent a year. During the 1973-74 oil crisis, travel fell for the first time since World War II. Then the growth resumed until March 1977, when the trend line dropped downward again.

Every month since last May, U.S. drivers have traveled fewer miles than in the same month a year earlier, except for January. Blizzards in January 1979 cut travel in much of the country, so January 1980 showed a slight gain.

But factoring out the weather, the drop has been continuous, and total highway travel for the first three months of this year was 2.9 percent below the first quarter of last year.