(Image: U.S. EIA via Climate Progress)

A 10 percent annual increase in U.S. transit ridership would reduce CO2 emissions by 180 million tons each year, taking the nation halfway to the target set by the House climate change bill within three years, according to a report [PDF] released today by Environment America and the Coalition for Smarter Growth.

The report, timed to coincide with the growing debate over transit’s role in the final version of the congressional climate bill, includes a wealth of useful and surprising data about how last year’s much-discussed rise in transit use translates into reduced driving and environmental benefits.

For example, that 10 percent increase in transit ridership is already happening in five states, all of which also saw a notable drop in vehicle miles traveled last year. And guess which five saw double-digit rises in ridership? Not New York or Massachusetts — but Louisiana, Idaho, Utah, Delaware, and Maryland.

"A lot of [transit] growth that we’re seeing isn’t in typical big cities," Environment America transportation advocate Rob McCulloch, a co-author of today’s report, said in an interview. "It’s in suburbs and smaller communities where people are opting in. We think that’s really where the opportunity is."

The report describes a 10 percent increase in transit ridership as a "high but realistic target," but it goes on to make a clear case for setting such a goal:

[I]n 15 years such an approach could reduce transportation oil consumption by 20 billion gallons per year — equivalent to what we currently import from the Persian Gulf. This would also result in an annual reduction of 180 million tons of carbon dioxide pollution — more than four times the current benefit conferred by public transportation.

That annual cut of 180 million tons of CO2 would amount to 3 percent reduction below 2005 emissions levels every year. The climate bill passed by the House in June aims to reduce emissions by 17 percent below 2005 levels over the next 11 years, making a national transit-ridership target a key weapon in the arsenal of climate policy-makers.

McCulloch and his co-authors make several policy recommendations to lawmakers now working on transport and energy proposals, but their most powerful message comes in the framing department.

At this month’s University of Virginia infrastructure conference, one popular lament was that transportation lacks a national "story," a coherent and catchy appeal to Americans from all walks of life. Bicycle and transit advocates may well disagree, as may state DOT officials who think of more roads as the be-all, end-all of infrastructure policy.

Yet it’s easy to see a "story" emerging from today’s transit report, one that’s focused on flexibility — for transit agencies to use federal money to keep operating and for officials to use funds on different modes of transport — as well as a common goal of reducing the nation’s expensive, crippling oil dependence. The more that lawmakers and environmental groups use those themes to make transportation a bigger part of the climate debate, the better.