Defying common wisdom that a sour economy keeps cars off the road, the number of vehicles puttering around the Washington region has increased since the Wall Street debacle three years ago.

More than 3.8 million vehicles now are registered in the District and adjacent parts of Maryland and Virginia, a 4 percent increase since 2008, according to a report that will be presented to the Metropolitan Washington Council of Governments on Wednesday.

This should not surprise anyone who has been on the Capital Beltway or any of the region’s other interstate highways, and anyone who has sat in traffic probably has noticed that they are keeping company with far more hybrid vehicles than were on the road a decade ago.

The surprise in the report is that the trend toward SUVs ended after 2004, when the split between them and passenger cars was about 50-50. Motivated in part by a need for greater fuel efficiency, 60 percent of drivers now own passenger cars.

“It’s sort of a values shift,” said Ron Kirby, the council’s transportation planning director. “Of course the earlier trend toward SUVs was a values shift, too.”

Since strict emissions controls for SUVs came years after they were imposed on cars, Kirby said, “we’ve still got some of the dirty ones out there.”

The report was prepared by the council’s staff for its Transportation Planning Board, a panel of elected and appointed officials from across the region. One of the board’s duties is tracking vehicle emissions, and the report draws on state motor vehicle data to determine the types and ages of vehicles registered in the region.

The report concluded that more vehicles meant more pollution, particularly pollution created by use of diesel fuel. Recent improvements in the quality of diesel, however, have reduced the impact the fuel might otherwise have had.

Although the number of hybrids increased each year except 2009, the grim economy has seen people holding on to cars a bit longer than they once did. That causes concern from an environmental standpoint, since newer models pollute less.

“The fleet is getting older, undoubtedly as a result of the recession, and getting an emissions reduction really counts on the fleet turning over so that we have newer vehicles,” Kirby said.

He said that cars on average are a year older than they were when the last study was done in 2008. SUVs are 18 months older and heavy trucks are two years older.

Besides calculating emissions by vehicle type, the board uses the information on whose driving what to track socioeconomic trends.

The report found that the number of vehicles in the Maryland and Virginia suburbs had increased by 4 percent since 2008, while there had been a 3 percent increase in the District.

Cars and motorcycles are more popular in the District (65 percent of the total), while SUVs account for 40 percent of the total in the suburbs. Trucks and buses are about 5 percent of the vehicle fleet in all three jurisdictions.

In addition to the District, the report includes Alexandria, Arlington, Fairfax, Prince William, Loudoun, Calvert, Charles, Frederick, Montgomery and Prince George’s counties.