Many drivers underestimate how fast some of the road bikes can travel.
Recently, I was biking at about 20mph in the neighborhood taking advantage of empty streets and perfect visibility, and could see in a distance (~100m) lady preparing to back into the driveway, she looked at me with flashing front light and neon bright shirt, hesitated, and obviously assessed that there was enough time for her maneuver, and she moved the car across the road blocking the passage. However, in a few seconds I was close enough that I had to break, no emergency, but surprise. I am pretty sure she would have waited if only she realized that I was coming so fast.