What is the "ridge of death" in cycling?

In Belgium and Flanders they have what is colloquially known as the “ridge of death” or “death ridge” on their roads. This is a thin center strip that connects the two road halves into one. Given the construction of the roads, which are two giants slabs of concrete, one for each lane of traffic, there’s a gap between the two where they just don’t quite overlap.

This gap is often filled in (or sometimes not filled in) with a different material than the concrete. When riders’ wheels touch this different material, or fall into a section that’s not correctly filled, it sends their bike off course. You’ll see riders always trying to ride on either one side or the other to avoid the dreaded gap.