Streetsblog reported last week that Park Slope’s Fifth Avenue Open Street, one of Brooklyn’s most popular, has lost its sponsor and is now in search of a new one in order to open as planned this year. Another, Vanderbilt Avenue in Prospect Heights, will be cutting its overall operating hours by 40 percent, reducing its season by two months (from May to September instead of April to October), and ending five hours earlier on Sundays. The programs, which close blocks to traffic to create more public space, have been celebrated, somewhat controversial, and undeniably successful. They are also expensive to maintain.