Drivers continually under estimate the speed of bicycles. I think it relates to their personal experiences in riding bicycles.
I have had a vehicle turn left right in front of me when I was going about 25 mph, even though I was clearly visible and I knew that he saw me. I anticipated what he was going to do and slowed to allow the turn. He underestimated my speed. I also said a couple of choice phrases, which really did not serve any good purpose except to vent my frustration.
One technique I use in driving and biking defensively is to watch the eyes of the driver, and the direction of the front wheels.