Up in Westminster/Broomfield, there are several actual bike tunnels that go underneath a main road e.g. on Big Dry Creek bike trail underneath Wadsworth Blvd.

Up in Westminster/Broomfield, there are several actual bike tunnels that go underneath a main road e.g. on Big Dry Creek bike trail underneath Wadsworth Blvd. The tunnels do get a bit wet/impassable if it has rained or snowed heavily, or if the nearby creek is running heavily (which is rare). But people dont typically bike in downpours. It doesn’t really matter, the point is that cyclists are totally separated from car traffic.