On Monday, New York City Mayor Michael Bloomberg unveiled a new event called "Summer Streets." For three Saturdays in August, pedestrians and cyclists will enjoy exclusive access to a contiguous stretch of city thoroughfares running from the Brooklyn Bridge to 72nd Street. No cars allowed. This goes way beyond street fair territory. We're talking about a path more than half the length of Manhattan -- 6.9 miles, to be precise -- where people can walk, bike, shop, and otherwise enjoy the city free from the intrusion of motor vehicles. The core idea behind Summer Streets, according to Transportation Commissioner Janette …