Similar story different ending, I was driving over the San Mateo Bridge in broad daylight and screeched to a stop to avoid hitting a bicycle that had fallen off the back of someone's car and was sitting the rightmost lane. We managed to get out of there after spending a long terrifying minute looking in the rearview mirror at vehicles blithely approaching us at 75 mph and swerving into another lane at the last second upon realizing we were stopped, despite the emergency blinkers. Finally a gap in traffic opened up and we were able to get out of there too.
After we cleared the bridge we called 911 and as soon as my girlfriend said the words "San Mateo Bridge" the dispatcher said "Are you calling about that bike? All the calls we're getting right now are about the bike. Police are already on the way. Have a good day." And she got hung up on!
So, fortunately, sometimes the bystander effect isn't in play, and lots of people are independently making the good decision to report a dangerous situation to 911.