Interviews with drivers and transit riders indicate, however, that a change in habits has not come easy  and might be reversed if gas prices fall.

“When prices went over $4, I stopped driving,” said Scott Pisciotti, 41, a real estate portfolio manager from Somers, N.Y. Mr. Pisciotti said he used to drive daily to his office in Midtown Manhattan but now rides a Metro-North train from White Plains to Grand Central Terminal. It is a new routine for him, and he has not fully embraced it. “If prices dropped,” he said, “I would drive a lot more.”

Commuter trains have also become more crowded, riders say.

At the Secaucus Junction train station of New Jersey Transit, Brian Simmons, 30, said that it had become much harder to get a seat on the train in recent months. “It’s like the New York City subway,” he said.

Gas price-induced traffic reduction might have a downside. Mr. Bloomberg’s plan was intended, among other things, to raise hundreds of millions of dollars a year for mass transit improvements by charging cars an $8 fee to enter the area of Manhattan below 59th Street. The plan was defeated in April when legislative leaders in Albany refused to bring it up for a vote.

In contrast, the current reduction in traffic at bridges and tunnels could actually take money away from transit, because a large portion of the tolls collected at the transportation authority’s crossings helps to finance the subways, buses and commuter railroads. In May, toll revenues were more than $4 million below budget projections, and Gary J. Dellaverson, the authority’s chief financial officer, said that June toll revenues appeared to be down even further.

So far, the drop has been more than offset by an increase in fare collections generated by higher transit and rail ridership, but Mr. Dellaverson said that the combination of slipping toll revenues and the increased cost of fuel for the authority’s buses and trains could eventually outpace ridership revenue gains.

The rise in gas prices may also be increasing something that congestion pricing was meant to eliminate: the incentive for drivers to avoid tolls by using the free bridges over the East River, causing heavier traffic in the surrounding neighborhoods.