D.C. Mayor Marion Barry yesterday nominated the departing executive director of the Carter administration's Urban Mass Transportation Administration to head the city's Department of Transportation.

Barry named Thomas M. Downs, 37, as the $50,112-a-year transportation director, a post that has been vacant since the retirement last April of the outspoken former director, Douglas N. Schneider.

Downs' appointment must be approved by the City Council, and, if he is approved, Barry said Downs will assume his new job on Jan. 5. In his current job, Downs has served as the chief administrative officer of the agency that dispenses federal aid for urban mass transit systems.

Since Schneider's departure, his deputy James E. Clarke III, has served as acting director, and has for the most part pursued Schneider's philosophy of discouraging downtown driving by making public transportation more convenient than use of cars. Clarke, who had hoped to become the permanent director, said he was disappointed at being passed over for the job and plans to leave the city government.

Downs, formerly city manager of Leavenworth, Kan., a small Great Plains town of 40,000, said yesterday that while he had a "deep and abiding love of transportation" he did not bring "a specific agenda that I come ready-equipped with.

"I am first and foremost a manager." Downs said. He said that his top priorities would be system management and "infrastructure maintenance" ?? that is, comprehensive and badly needed repair work for the city's streets and bridges.

"That means spending a lot of money to repair roads and bridges," Downs said. "You've got to keep intact what you've already got in place. It's a significant investment that hasn't been made in a couple of years." Downs said he already has Barry's support for such a program, although finding the money might be difficult because of the city's continuing budget problems. Such a program could amount to a massive reconstruction of many city streets.