From the latest revision on SFGate:
The crash happened near 16th Street and South Van Ness Avenue about 6:45 a.m. when the truck, which was parallel to the bicyclist on eastbound 16th, tried to make a right turn onto southbound South Van Ness, police said.
The cyclist was riding "at a high rate of speed" and hit the back of the turning truck, police said.
The driver continued a short distance on South Van Ness, said the witness, 29-year-old
Jorge Marquez of San Bruno, but stopped after people on the street alerted him to what had happened.
The picture may be getting clearer. Neither party appears to have been operating properly in this version.
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"What if we fail to stop the erosion of cities by automobiles?. . . In that case, we Americans will hardly need to ponder a mystery that has troubled men for millennia: What is the purpose of life? For us, the answer will be clear, established and for all practical purposes indisputable: The purpose of life is to produce and consume automobiles."
~Jane Jacobs, The Death and Life of Great American Cities