Everybody knows traffic in the Washington region is terrible. So let’s look at how the House Republicans’ transportation plan would affect it.

The overall impact could be pretty severe.

The GOP-controlled House has proposed to reduce federal transportation spending by nearly a third, about $15 billion a year, over the next six years.

Put simply, that proposal would stall or reverse our region’s plans to improve and expand mass transit and roadways in coming years, according to many officials and private experts in Maryland, the District and Virginia.

Like most places around the country, all three area jurisdictions depend heavily on federal dollars to help finance transportation investments.

“We would have to go in and remove projects we have been counting on. There would be precious little funding available to do anything new,” said Ron Kirby, the veteran transportation expert at the Metropolitan Washington Council of Governments.

The Metro transit system said it stands to lose about $70 million a year under the GOP plan. Chief Executive Richard Sarles has warned that the loss of federal funds would mean more delays, less reliable trains and buses, and deteriorating stations.

In suburban Maryland, there’d likely be an indefinite postponement in building the Purple Line light-rail link between Montgomery and Prince George’s counties, and the Corridor Cities transitway (light rail or express buses) parallel to I-270 in Montgomery.

In the District, it would be harder to find funds to replace the structurally deficient Frederick Douglass bridge next to Nationals Park, and to expand the coming streetcar network.

In Virginia, less money would be available for future plans to expand road and transit capacity along the I-66 corridor outside the Beltway, and along I-95 south of Stafford.

Among the people I interviewed, the only one who minimized the potential problem was Virginia Transportation Secretary Sean Connaughton. The Republican said it would be possible to offset the loss of federal money by reducing bureaucracy and relying more on federal loan programs and public-private partnerships.

On the downside, such steps would almost certainly mean a sizable jump in state borrowing, or tolls or both. Also, other experts were less confident that Virginia could manage the problem so readily.

“Any decrease in federal funding would have a significant impact across the board in Virginia on highway construction and mass transit,” said Bob Chase, president of the Northern Virginia Transportation Alliance.

Considering how many hours we waste in traffic backups already, it would be a major setback to give up whatever momentum we have in reducing congestion. Moreover, we’d be ignoring the need to add capacity to handle the hundreds of thousands of new residents expected to swell the region’s population in the years ahead.

Of course, there’s a decent chance that the Democratic-led Senate and President Obama will force the House to yield on some of the transportation cuts.

Still, with the anti-spending mood so strong on Capitol Hill, it’s safe to bet that our region will have to make do with less than it’s been expecting. There’s no appetite for the most obvious solution, which is to raise the federal gasoline tax, unchanged since 1993.

U.S. Rep. John Mica, the Florida Republican who chairs the House transportation committee, said the spending cuts needn’t yield pain. Trimming federal programs and streamlining requirements would produce savings, he said, and partnerships with private business would provide leverage to maximize the impact of dollars still available.

Chase welcomed such changes but said they wouldn’t be enough.

“It’s good to reform the structure, but if you don’t have adequate money going through that structure, then the death spiral is just prolonged. At the end of the day, the result is the same. You have a transportation system that doesn’t work,” Chase said.

Keep that in mind while following the debate over slashing spending. You’ll have plenty of time to listen to the radio news as you creep along the Beltway.

I’m taking a break. Column resumes July 21.