Has anyone seen these black, rectangular 'dots' taped to the road surface lately? They are located about a mile apart, on highway 16 between Plymouth and Sloughhouse in Northern California.

They aren't much of a hazard, but I wonder what the heck they are. They are all placed in the center of the travel lane. You will probably miss them if you are in a car, but if you are on a bike, you may ride over them.