H/T to Donna!
In my part of the Midwest, those were equally interspersed with "Concord grapes for sale" signs. Or watermelons, or cantaloupes, and corn in the fall.
A correct answer years and years ago would be Burma Shave.I don't think Burma Shave actually put this one up. Beer cans lying by the road,are unsightly so some say.But at night, reflecting bright,they safely guide the way.