Could fewer cars on the road jeopardize public transportation?

The US Department of Transportation warned on Friday that a 10-month decline in traffic on American roads could jeopardize not only new highways and bridges, but also improvements to mass transit systems.

"We pay for transit the same way we pay for road and bridge projects: with federal gas taxes," Secretary of Transportation Mary Peters said during a visit to a light-rail system now being built in Dallas. According to a press release, the Department of Transportation wants to "make it easier to attract new souces of funding for transportation projects."

Could that mean a tax on mass transit fares, based on the same gas-tax principle that users of a service should help to pay for it? Or does it mean a new revenue stream that isn’t necessarily linked to the service itself — such as the penny of each five cents in sales taxes that goes to the MBTA in Massachusetts?

The DOT estimates that Americans drove 15 billion fewer miles in August 2008 than they did in the same month a year earlier. The 5.6 percent decrease was the largest year-to-year decline ever recorded for a single month, and it marked the 10th consecutive monthly decline.

The drop in traffic was sharper on rural highways than on city roads, which helps explain why traffic in heavily urban Massachusetts fell by only 4.4 percent. The sharpest decrease was in Florida (down 9.7 percent), followed by Alabama and Georgia. Traffic in the District of Columbia rose by 3.2 percent, and it dropped by only 1.0 percent in North Dakota and 1.2 percent in Pennsylvania.