I looked up the amusing “glad rooster” and “wild rooster” warning signs I’d seen on the trip, and Google Translate says they mean “smooth grid” and “cattle grid”.
Aha! That seems believable, I think I generally saw them near cattle grids, those open-bar grates that cows won’t cross but humans can (as well as bikes, if they’re careful to stay perpendicular to the slots– the slots are certainly wide enough to eat a wheel).
Example image from a web search: