SAN DIEGO — A motorcyclist fractured both legs Sunday night when he crashed while driving on a sharply curving transition ramp in Mission Bay, police said. It happened about 8:45 p.m. on the ramp from West Mission Bay Drive to Ingraham Street, according to San Diego police Officer Sarah Foster. The 30-year-old rider lost control and hit the concrete wall, Foster said. Medics took him to a hospital for treatment of his serious injuries. The San Diego Police Department’s traffic division is investigating the crash.