There are definitely many variable-tolled roads. The trouble is that frequently they are embedded in a system of non-tolled (or, really, not-allowed-to-be-tolled) roads, and that the tolling is not well-focused on congestion. Basically, the Braess "paradox" emerges if you try to make a road network that's optimal for travel patterns over an entire day, without using pricing to ration scarce space.