Does the recent decline in Americans' driving mark a transformation in how we get around, or is it just a speed bump on the highway to ever-greater traffic?

A study released today by the Brookings Institution concludes that the drop-off represents a "permanent shift from reliance on the car to other modes of transportation."

Other experts are unpersuaded. They say the decline is likely temporary, created first by high gas prices and now by the nation's worsening economy.

The Brookings study found that the total vehicle-miles traveled in the United States fell by less than 3 percent between December 2006 and September 2008. The period was marked by sharply rising gas prices, but the Brookings authors said the decrease in driving was more than just a reaction to the cost of fuel.

The authors cited a number of other factors that affected driving: market saturation of vehicle ownership; a leveling off of the number of women entering the workforce; increased ridership on mass transit; rising unemployment; development of commercial centers closer to workers' homes; migration of baby boomers from suburbs to cities; and a possible ceiling in the amount of driving an individual can tolerate.

Pennsylvania and New Jersey showed smaller percentage declines than the national average, with 2.5 percent and 1.7 percent, respectively. The Philadelphia region was 17th lowest among 100 large metro areas in per-capita driving.

Recent numbers from the Federal Highway Administration show the decrease in driving has continued even as gas prices have fallen sharply. In October, the most recent month for which data are available, vehicle miles traveled were down 3.5 percent from a year earlier.

"It's not possible to say exactly why, but it's clear that something larger is going on in America," said Robert Puentes, an author of the study. "There's no doubt that the economic downturn is a very important factor . . . but we detected these trends beginning years ago."

In the last two years, mass transit has recorded the highest ridership numbers since the 1950s. Amtrak ridership reached a record this year.

In the Philadelphia region, SEPTA and PATCO both showed increases in transit riders, and for the first time in nearly a half-century, Center City vehicle traffic dropped in 2005, according to the most recent survey by the Delaware Valley Regional Planning Commission.

The Brookings authors said the apparent change in driving habits should be a factor in national policy-making on energy and the environment, as well as transportation. They recommended an increase in the federal gas tax to help pay for highway and mass transit improvements.

Others suggest the decline in driving may not last.

"I have a lot of difficulty believing any of it," said Alan Pisarski, a traffic consultant and author of the book

Commuting in America.

He compared the current decline to drop-offs in the early 1970s during the oil embargo and in 1979-80 during the Iranian revolution.

"As gas prices shifted lower or the economy improved, people went back to the growth rates," he said. "The automobile still generates an immense benefit for very many people, and they are going to continue to take advantage of it."

David Ellis, a research scientist at the Texas Transportation Institute, said, "I don't know that we can say that this is permanent yet. We've had declines like this in the past, and we've seen [vehicle-miles traveled] come back."

Several things are likely at work, said Barry Seymour, executive director of the Delaware Valley Regional Planning Commission. "What remains to be seen is whether people find transit a reasonable alternative and stick with it as gas prices come back down."

Scott Brady, manager of travel monitoring for the DVRPC, predicted that unless gas prices stay high for many years, Americans will not make permanent changes in their driving habits.

He said the recent spike in gas prices "got a few people thinking, and that's important. If we can turn that thinking into action and present people with real travel options, then we'd see change."

Contact staff writer Paul Nussbaum at 215-854-4587 or pnussbaum@phillynews.com.