I'll throw my support behind the designs from 103, 1519, and 2134 mentioned in this thread...
Us at 3710 have had some good results with a ramp-mechanism. Ours is deployed with a 4 bar mechanism to save a bit of space, but also to let mechanical stops and the chassis to bear the forces of impact with the bridge.
This has worked pretty well for a plywood test bridge (built by our buddies at 2809) and will see competition this week at Montreal - we can take the bridge at about half of our maximum speed.
We've whitepapered the design and made our CAD available - it's mostly aluminum angle and ours is bolted right on to a Kitbot on Steroids. Available here:
http://www.cyberfalcons.com/whitepapers/