The selective rain that seems to hit only the actors is not falling on the background, which remains dry.
At the beginning of the film, George and Faith Dunlap drive over the Bay Bridge to get to San Francisco. It's most likely they would use the Golden Gate Bridge. Its north terminus is the Marin Headlands in Marin County. To take the Bay Bridge, they would have to cross the Bay on 580, then travel south through Richmond, Berkeley and Oakland to reach the Bay Bridge, then head west to San Francisco.