It has been a long-held dream of New Yorkers of a certain (greenish) stripe: the streets of Manhattan free of cars. Now, for a few hours, on a few streets, on a few weekends this summer, that dream will become reality.

Mayor Michael R. Bloomberg announced on Monday that he will create a car-free zone on three Saturdays in August, along a 6.9-mile stretch of streets through Manhattan, from the Brooklyn Bridge, north to Park Avenue and the Upper East Side. Cars, trucks and buses will be banned on the streets along the route from 7 a.m. to 1 p.m. on Aug. 9, 16 and 23. The mayor was careful to describe the initiative, called Summer Streets, as an experiment.

“If it works, we’ll certainly consider doing it again,” Mr. Bloomberg said, at a news conference in the East Village on Lafayette Street, which will be included in the route. “If not, we won’t. But we have never been afraid to try new ideas, especially the ones that have the potential to improve the quality of life.”

The route will run north-south along Centre Street, Lafayette Street, Fourth Avenue and Park Avenue to 72nd Street. The southern half of 72nd Street from Park Avenue to Fifth Avenue will also be shut to vehicles, to link to Central Park.