Government regulators are planning to clamp down on diesel trucks, buses, and other vehicles, which make up only 2.5 percent of vehicles on the road but account for 26 percent of nitrogen oxides and up to 70 percent of soot in urban air. Federal rules permit diesel vehicles to emit much more pollution than standard passenger vehicles. Some states are moving ahead on their own rather than waiting for the feds to act. Last month, nine northeastern states agreed to conduct roadside tests on diesel trucks, ticketing offenders and forcing them to get repairs. California’s Air Resources Board has decided to classify diesel-exhaust soot as a cancer-causing “toxic air pollutant,” which could lead to lawsuits against the industry from people who claim exhaust has made them sick. But the trucking industry and its allies are formally asking the state board to reconsider the classification.