More than one third of Maryland's roads, including sections of the Beltway and other Interstate routes, are "substandard" and so rough that driving on them is costing motorists an extra $373 million a year, according to a study funded by the Maryland Highway Contractors Association.

Using state highway department data and its own surveys, the study says 3,228 miles of the state's 8,843 miles of main paved roads are substandard, with 1,128 miles of them in such poor or very poor condition that the riding quality is "intolerable." The remaining 1,990 miles of substandard roads are in fair condition and "tolerable" to ride.

Using a federal study on the costs of driving on substandard roads, the study concludes that it is costing motorists almost 50 percent more in fuel, or $373 million a year at current gasoline prices, to drive on substandard roads. Frequent slowing to avoid bumps and potholes followed by acceleration causes cars to use more fuel than they would use them when driven at even speeds on smooth roads, the study says.

But M. Slade Caltrider, head of the State Highway Administration, questions both the study's findings on the condition about the gasoline that could be saved if the roads were improved.

"I agree with what they're trying to do . . . we desperately need more funds for state roads, but I disagree that one-third of the roads are in physically bad shape. And we can't substantiate the gasoline savings" the study claims could be made if the roads were improved, Caltrider said.

Washington's 20-year-old Beltway is substandard in at least two Maryland sections, according to the study: between the Woodrow Wilson Bridge and Indian Head Highway (Rte. 210), and the highway's two original outside lanes between University Boulevard (Rte. 193) and Georgia Avenue (Rte. 97). Caltrider said these sections "badly need repair," but they are not in "intolerable" condition.

The study also cites as in poor or very poor condition sections of Georgia Avenue from the Beltway to Wheaton, University Boulevard from Wheaton to Four Corners (Colesville Road), Landover Road (Rte. 202) between Cheverly and George Palmer Highway (Rte. 704), Silver Hill Road (Rte. 458) south of Rte. 4, Route 3 in Anne Arundel County and Pennsylvania Avenue (Rte. 4) east of the District line.

The study was prepared by The Road Information Program (TRIP), a nonprofit group funded by insurance companies, auto manufacturers, highway contractors and companies in highway construction.