Americans are notoriously reluctant to forsake their automobiles and ride public transportation, but a new study shows that skyrocketing gas prices are changing behavior. The American Public Transportation Association reports that ridership on public transportation rose 3.3 percent in the first quarter of 2008 compared with the same period in 2007, and that the number of commuters riding light rail systems and commuter railways was up by double digits in many cities. The increase reflects a trend that began in 2007 as gasoline prices edged up steadily and then crossed the $4-a-gallon threshold this spring. The association reported that in first three months of 2008 ridership on commuter rail rose by 28 percent in Seattle, 17 percent in Oakland and more than 10 percent in Philadelphia. Ridership on light rail — trolleys and streetcars — increased by 17 percent in Baltimore and 16 percent in Minneapolis and St. Louis. Subway ridership in large cities was up 4.4 percent, and bus ridership increased by 8 percent in communities with less than 100,000 people. During the same period, the Federal Highway Administration said, the number of miles Americans traveled by road declined by 2.3 percent.