Not as many since they banned them here, but it used to be quite common given the combination between the fact it's pretty windy here in SF every afternoon, and there's lots of trash / debris around. It's still pretty common to see blowing paper or other debris (tire shreds) in the road at least a few times in my 12 mile commute.

This has been a not-minor problem for autonomous cars and the Tesla-style autopilots / adaptive cruise controls that depend on vision only. You have to program it to ignore some types of things that seem like they might be an obstruction, such as road signs, debris in the road, etc. so they don't hit the brakes unnecessarily.