With the land set to 1ft and a river path laid down on it, I set my bridges to -90.
This also makes for a good angle to run a road all the way up to meet the road bed on the bridge and you can set the segment point elevations so that you have no road shoulder on the bridge berm. I discovered with Hitech introducing vehicle bounce on uneven terrain clutter, if the road shoulder is present on the end berm for a bridge. Your vehicle has to be on the road center when you start climbing up the berm to the bridge bed. That tiny bit of shoulder by default, if your vehicle was climbing the berm from an off angle from the road up to the bridge road bed. When the front quarter of the vehicle bounced upon meeting the shoulder to run up on it, part of the time the vehicle was tipped over on it's side rolling off the berm.
I'm halfway through lowering all of my bridges on the terrain to -90 and resetting the elevations of the road segments that run up to and meet the road bed from each end.
Hitech was very right, paths are irritating.