"Here comes the lady on the mean machine," an old man on the corner, yelled as Cecile Roesch putt-putted along Columbia Road NW one recent morning.

Roesch, a Catholic University student, is one of hundreds of gasoline-hungry Washington-area consumers who have given up their cars recently for "mean machines" called mopeds - bicycle-sized motor bikes with engines so tiny they get as much as 150 miles per gallon.

As area dealers report they are selling mopeds as fast as they can get them, police in the District of Columbia and Fairfax County say thieves are stealing the motorized bikes as fast as they can find them.

"The day of the moped is here," says D. C. Detective Clive Pulsifer. "Because we're knee-deep in them and because there is an energy shortage, everybody wants one. When there's a market like that, what do you expect thieves to do?"

Roesch, who says she hasn't sat in a gas line since buying her moped, travels a 12-mile route from her home to her job at the Washington Hilton Hotel and on to summer school in North-east. She gets about 100 miles a gallon.

Dorothy Hughes squeezes even more out of the $560 moped she bought to beat the gasoline shortage. "I get 150 miles on one gallon," she claims.

Mrs. Hughes, 52, rides her vehicle 10 miles each day from her home in Boulevard Heights, Md., to her job at Langdon Elementary School. She is a teacher assigned this summer to a special curriculum planning program there.

"Two of my coworkers now are thinking about buying mopeds now that they have seen what mine will do," Hughes said.

Local dealers say such mileage figures have helped create an upsurge in sales since long lines appeared at area gasoline stations about two months ago. Jim Metzler, manager of a Rockville moped dealership, said, "We sold 30 bikes last June. This June we've sold 150." Metzler says he finds "a new energy consciouness" among his customers.

The energy crunch also has changed the moped constituency. While teenagers previously accounted for the bulk of his moped business, Frank Young of Young Motor Co. in the District says more than half his recent sales were to adults.

Despite dealer enthusiasm, legal obstacles remain for some metropolitan-area moped riders.

Maryland requires owners to have an automobile or motorcycle operator's license. In the District, mopeds, like cars, need a license, tags and periodic inspections.

Maryland and Virginia riders may be ticketed in the District, according to police, unless they have an operator's license, proof of residency or proof of ownership.

Officials in the three areas say no changes in these laws are planned unless they learn of new information concerning accidents or safety hazards.

While specific figures are unavilable, a University of North Carolina study for the National Highway Traffic Safety Administration projected a three-fold rise in moped-related injuries by 1984.

But, "they're fairly safe - safer than motorcycles," says Herbert Miller, a research psychologist with the traffic administration.

Meanwhile, mopeds are catching the eye of thieves as well as commuters. "We noticed a surge in reported thefts just after the gasoline lines formed," says Fairfax police spokesman Warren R. Carmichael.

Detective Richard Manacle of the District said one precinct station had nine recovered stolen mopeds in its back lot recently and that one youth he knew of was victimized twice by the same moped thief. "The first time we recovered it in the District," Manacle said. "The second time we recovered it in Maryland."

District detective Pulsifer says he expects thefts to increase during the summer. "We've got a real hot item on our hands," he says. "People will either lock them up or expect them to be stolen." CAPTION: Picture 1, Dorothy Hughes says she gets 150 miles to the gallon on her moped. By Gerald Martineau - The Washington Post; Picture 2, Dealers report an upsurge of sales since start of the gasoline shortage. By Linda Wheeler - The Washington Post, Picture 3, Cecile Roesch, a student at Catholic University, has given up her car for a motor bike. By Fred Sweets - The Washington Post