Tim Lomax, who does those mobility studies that we love to quote because they show the D.C. region is a national leader in bad traffic, once explained to me what sets us apart: Well, he said, you’ve got two rivers.

For years, advocates for regional transportation improvements have been trying to figure a way around that, with very little luck. Usually, we settle for making the existing bridges wider, or at least safer.

Metro’s Rush Plus plan, the shifting around of rush hour trains, basically will shift trains from one Potomac River crossing to another. The District Department of Transportation is helping out this year by expanding the capacity of the 11th Street Bridge over the Anacostia River.

But what some advocacy groups long for is a new Potomac River crossing to the northwest of the American Legion Bridge. The advocates want to ease congestion on Interstate 270, the Capital Beltway and the Dulles Toll Road.

They were encourage recently by a rather mild expression of support from Virginia Gov. Robert F. McDonnell (R) and Maryland Gov. Martin O’Malley (D).

But the reality, as O’Malley acknowledged later, is that any action on this will occur after their terms expire. McDonnell’s term is up after 2013 and O’Malley’s is up about a year later.

On WTOP radio’s “Ask the Executive” show today, Montgomery County Executive Isiah Leggett took a position similar to O’Malley’s by saying that any decision on a river crossing would be in the hands of a successor generation of government leaders.

That sort of talk bothers Mahlon G. (Lon) Anderson, the managing director of public and government affairs for AAA Mid-Atlantic. He’s one of the advocates for a new crossing.

“It’s nice to hear that Governors McDonnell and O’Malley acknowledge that another bridge across the Potomac is needed, but their support seems tepid at best and that is extremely disappointing,” Anderson said in a statement this week.

“The fact is that we need a new way for commuters to get to and from their homes and workplaces in Maryland and Virginia,” he said. “It’s encouraging that Governor O’Malley sees the need for a new bridge, but discouraging that he says this is not something he foresees in the near-future.”

The Northern Virginia Transportation Alliance and The 2030 Group on the Maryland side have been supportive of new crossings to relieve traffic.

But it just isn’t happening. There’s no serious proposal on the table. Virginia is building the high-occupancy toll lanes on the Beltway and the Metro Silver Line through Tysons and out toward Dulles International Airport. Maryland is considering improvements to the Beltway north of the Legion Bridge, as well as some way of adding capacity to I-270.

That’s about it. Don’t look for anything more unless you can figure out where the government has stashed a few billion in loose change, and you have a way to anesthetize the communities on both sides of the river that will raise a huge stink once any leader gets brave enough — or crazy enough — to say, “Put the bridge right here.”