Heavy diesel vehicles are the major culprits behind the vehicle exhaust problem that threatens federal funding for new road and transit projects in the Washington area, regional planners told public officials yesterday.

Tractor-trailers and other such vehicles make up just 3 percent of the traffic on the region's roads, but they belch out about 30 percent of the pollutants that contribute to unhealthy levels of ground-level ozone, planners said. By comparison, passenger cars compose about 40 percent of local traffic but produce 17 percent of the exhaust.

Sport-utility vehicles and light trucks make up about half of all traffic but about one-quarter of emissions.

Regional transportation experts said it's the first time they have been able to analyze in such detail what kinds of vehicles are most responsible for the jump in unhealthy exhaust.

The huge amount of exhaust coming from the relatively few diesel trucks "just hits you right over the head," said Mike Clifford, who studies transportation-related pollution for the Metropolitan Washington Council of Governments.

COG planners and regional officials were startled in November, when they discovered that the pollution generated by the region's crushing traffic was far worse than previously believed. The increase in SUVs, pickups and diesel trucks, they found, would cause the region to exceed its limits on exhaust by 30 percent in 2005.

The problem must be solved by January 2004, when the the current regional transportation plan expires, or any road or transit projects not already under contract would not win federal approval.

Members of COG's Transportation Planning Board, which heard the findings yesterday, said they will look first at replacing or updating diesel vehicles in government fleets, such as school and transit buses, garbage trucks and emergency equipment. They said they also would look at incentives, such as tax breaks, for the private trucking industry.

"We need to target these heavy-duty trucks," said D.C. Council member Phil Mendelson (D-At Large), the board's chairman.

"This just jumps off this chart, that such a small percentage of the vehicle fleet produces such a large percentage of the nitrogen oxide emissions," said board member Sean T. Connaughton (R-At Large), chairman of the Prince William Board of County Supervisors.

The most recent calculations also showed that although the region will far exceed vehicle exhaust limits in the next three years, it should not have a pollution problem in the long term, Clifford told the board. That's because new federal rules that take effect in the next five years require diesel trucks and all light trucks and sport-utility vehicles to be less-polluting.

The Washington region has violated federal clean air standards for years. The U.S. Environmental Protection Agency has proposed downgrading the region from being in "serious" violation to being in "severe" violation of those standards. Vehicle exhaust combines with sunlight to form unhealthy levels of ground-level ozone, which leads to the Code Red alerts that warn children, the elderly and people with breathing problems to limit time outdoors.

Local officials set their own limits on vehicle exhaust and other sources of emissions, such as power plants and construction equipment, as part of a larger plan to help the area meet federal air quality standards.

Because the recently discovered vehicle exhaust problem is so much worse than anticipated, local governments will have to pay for clean air programs that will have a large impact quickly. Complicating the problem, governments across the region also are facing severe budget shortfalls.

Last year, the region pledged $42 million in clean air programs over three years to curtail three excess tons per day of projected pollutants. This year, officials are looking at a problem of 50 tons.

Clifford said COG planners have not yet estimated how much measures to reduce diesel emissions would cost. Almost all public school buses and most transit buses run on diesel, he said.