Looking at a map of the Jubilee River (The Thames flood channel near
Windsor) it occurs to me that some of it would have made a useful road.
So I'm thinking that quite a few bypasses could have been built
deliberately low and could then operate as roads most of the time but as
flood alleviation channels when required. I'm wondering if this has been
done anywhere. Various films have included car chases in the Los Angeles
River, which I believe is dry most of the time, but they seem not to use
it as a road.