This weekend marks the start of New York City's experiment in car-free zones. For three Saturdays in August, a 6.9-mile stretch of streets through Manhattan -- from the Brooklyn Bridge to 72nd Street -- will be free of cars from 7 a.m. to 1 p.m. The initiative, Summer Streets, is aimed at giving New Yorkers and tourists a chance to explore and enjoy a popular throughfare on two feet or two wheels in a healthy, sustainable way. Fitness, dance and yoga classes will be held at a central stage along the route, with additional exercise and health activities hosted by community groups at select cross streets. Bike rental facilities will be also be available. Several other cities, including El Paso, Texas, and Cambridge, Mass., have adopted similar temporary car-free corridors; Portland and Chicago are said to be planning similar initiatives.