On Sunday, Time's Up! the East Village bike advocacy group, organized a five-borough ride to memorialize the 21 cyclists killed in traffic accidents in New York City in 2005. One hundred fifty cyclists from all five boroughs participated.The procession started in the outer boroughs with the cyclists riding to locations where bike riders had been killed by motor vehicles. By late afternoon a group of 40 cyclists had gathered at the Manhattan side of the Williamsburg Bridge and headed Downtown. Shortly afterwards, a large group of riders could be seen crossing the Brooklyn Bridge, heading into Manhattan. The two groups converged near City Hall and headed to their first Manhattan stop at Water St. and Governeur Ln. It was here that, on Sept.16, Jen Shao, 65, from Chinatown was killed by a private bus while riding her bike.By late afternoon, the mass had pedaled north, making their way along Houston St. and stopping at Avenue A to pause and lay flowers at the spot where Brandie Bailey, 21, was struck and killed by a garbage truck while riding her track bike home to Williamsburg on May 8. At the time, the truck driver, who later claimed he didn't know what had happened, continued driving north for over 20 blocks until finally being pulled over by police. Bailey commuted to work by bike, as so many others do. She was a waitress at the Red Bamboo Vegetarian Soul Caf