I took our kids to find one of these ("ecoducts" in the Netherlands) about a half hour drive from where we live. It was fantastic: no sign of humans anywhere except the speeding traffic under us on the motorway; just lots of animal footprints, and a single surveillance camera that I imagine is used to check out the furry users of the bridge rather than for any "security" related tasks.

The kids loved it :) They found it pretty spooky though.

It's very weird standing on what feels like a big hill with grass, sand and trees, knowing the motorway is rushing by under your feet.