Some cities with long-established public transit systems, like New York and Boston, have seen increases in ridership of 5 percent or more so far this year. But the biggest surges  of 10 to 15 percent or more over last year  are occurring in many metropolitan areas in the South and West where the driving culture is strongest and bus and rail lines are more limited.

Here in Denver, for example, ridership was up 8 percent in the first three months of the year compared with last year, despite a fare increase in January and a slowing economy, which usually means fewer commuters. Several routes on the system have reached capacity, particularly at rush hour, for the first time.

“We are at a tipping point,” said Clarence W. Marsella, chief executive of the Denver Regional Transportation District, referring to gasoline prices.

Transit systems in metropolitan areas like Minneapolis, Seattle, Dallas-Fort Worth and San Francisco reported similar jumps. In cities like Houston, Nashville, Salt Lake City, and Charlotte, N.C., commuters in growing numbers are taking advantage of new bus and train lines built or expanded in the last few years. The American Public Transportation Association reports that localities with fewer than 100,000 people have also experienced large increases in bus ridership.

Image Rod Crane, right, and other commuters use Denvers rail line. Credit... Kevin Moloney for The New York Times

In New York, the Metropolitan Transportation Authority reports that ridership was up the first three months of the year by more than 5 percent on the Long Island Rail Road and the Metro-North Railroad, while M.T.A. bus ridership was up 2.3 percent. New York City subway use was up 6.8 percent for January and February. Ridership on New Jersey Transit trains was up more than 5 percent for the first three months of the year.