Pedestrian Gravity
The law of physics which dictates that a slow pedestrian who is about to be overtaken by a fast pedestrian will suddenly and without warning swerve into the path of the fast pedestrian as if they are attracted to each other by real gravity. The slower pedestrian is usually oblivious to their surroundings and is quite dumb. The effect of Pedestrian Gravity increases exponentially with speed, with near or actual accidents occuring at high rates of speed.
I was hustling home along a busy sidewalk to take a dump but I didn't account for Pedestrian Gravity. This oblivious moron swerved right into my path and I bumped into her. We both fell over and I almost crapped my pants.