In North America, green bridges are generally limited to those highway overpasses in the Rockies designed to maintain habitat linkages; ideally, bears and other large animals cross the naturalized, tree-covered bridges over the Trans-Canada highway without too much disruption to their habitat. If you're not creating a safe passage for the preservation of diesel trucks, though, there's often little incentive to build such things - though their design and engineering is not complex.
In the Netherlands, development density (and a latticework of roads) is seriously compromising the movement of both fauna and flora, since small animals spread plants by transporting seeds inside their bodies and on their fur. While there appears to be an ongoing effort in the UK to preserve (for example) badger habitat with small ecoducts and tunnels, the Dutch province of Noord-Holland is addressing this issue with an enormous construction project. Spanning a railway line, business park, river, and sports complex, the Natuurbrug Zanderij Crailo (Crailo sand quarry nature bridge) will be the largest ecoduct in the world, a suburban conservation effort that has little in common with Alberta's occasional bear bridge.
The Crailo project stands alone in that it accounts for inevitable human traffic, while ambitiously working to reconnect ecosystems. Although green bridges turn up in urban areas such as London's Mile End Park, they have more to do with urban parkland than habitat integration. New York's High Line redevelopment will knit a landscaping and reclamation effort into an old elevated train structure, and I'm anxious to see it develop: the amazing Diller Scofidio + Renfro won the competition, and you can see the four finalists' visions via Friends of the High Line. The post-industrial salvaging reminds me a little of Germany's Park Duisberg Nord, which transformed a sprawling factory ruin into a huge urban park, retaining all its character while greening it in the most unexpected ways.
