In my locale, there's been extensive construction of bike infrastructure. There's some kind of grand plan, but it makes sense that provisions for bikes are adde

In my locale, there's been extensive construction of bike infrastructure. There's some kind of grand plan, but it makes sense that provisions for bikes are added when they're going to rebuild everything: The road, crosswalks, sewers, etc. And there's no space left in town where they're not trying to cram everything together. So the choices of bridges or tunnels tends to be one of what will actually fit. There's one giant road intersection where there are two bridges and a tunnel. The tunnel fits entirely within the footprint of the road above it, which is 6 lanes and carries heavy trucking. Its cost was probably absorbed by the overall project cost and need to support those 6 lanes throughout. The bridges can be much lighter, since they really only need to support themselves, but need extra space at the two ends for the ramps. One of these ramps displaced a house that was there. In the town where my son attends college, there's a bike path along a river, that regularly gets flooded when the river rises. But that was by design -- space for the bike path also doubles as a flood control measure, as is true of a lot of park space that's built along rivers.