The thing I HATE is tunnels - those that go under rivers for example. At the bottom they usually have drains in the middle of the road and over time the road has been resurfaced - except, of course, the drain. Which now sits down in an impression that is 1~4" deep - and the tunnel, being dark to some extent, and the rider wearing sunglasses, well... BUMP! And not a single sign on the side of the tunnel warning bikers to expect that dip in progression.
My 2¢.
