A national group of civil engineers said Tuesday that nearly all of the District's roads need significant work -- and that the nation's overall infrastructure had improved only slightly since 2009.

The American Society of Civil Engineers rated 99 percent of roads in D.C. as "mediocre" or "poor" in its "2013 Report Card for America's Infrastructure."

Robert Victor, chairman of the committee that created the report card, said the road quality statistics in D.C. were "nothing to be alarmed about" in terms of safety.

"A lot of the road quality has to do with rideability," Victor said. "What we're talking about here [are] really not things that are safety issues, but they are things that do cause mechanical problems with cars."

The report defined "mediocre" roads as those that generally require attention, while "poor" roads are below standard and have a "strong risk of failure." Spokeswoman Lynn Wallace said the road statistics were based on the most recent urban pavement conditions reported by the Federal Highway Administration in 2010.

The group of engineers analyzed data that had already been collected in each state and gave the United States' overall infrastructure a letter grade of D+, a slight improvement from a D grade in 2009, the last year the group released a report. Victor said he saw some encouraging signs in some of the report's 16 categories, such as the U.S. solid waste system, freight rails and bridges, but emphasized the need for increased investment.

"There's a direct link between our economy and our infrastructure," Victor said.

The report also said that 30 of the 239 bridges in D.C. are structurally deficient, which means they require "significant maintenance, rehabilitation, or replacement" and should be inspected at least once a year.

The District Department of Transportation disputed the report. Spokeswoman Monica Hernandez said 18, not 30, bridges were structurally deficient -- and that the report's high rate of poor-quality roads was also incorrect. Hernandez said the agency's own survey showed that 30 percent of D.C. roadways were in "fair" condition and 22 percent were in "poor" condition.

Beth McGinn, a spokeswoman for the American Road and Transportation Builders Association, connected the engineers' road quality findings with congestion in the D.C. area.

"I would say that D.C. is not only the capital of political gridlock, but our area is the capital of gridlock, period," McGinn said. "The public and private sectors have been saying this for years, that we need more investment in our nation's roads and bridges."

The state-by-state report also rated 47 percent of Virginia roads as poor or mediocre, along with 55 percent of the roads in Maryland.