Regarding the legal stuff, another thing would be to check out the wording on signs at a ski slope in your state. It's a pretty good bet that any liability sign at a ski slope has been past a legal department.
Edit: one thing about the sign below, I've known two people who are color blind, neither could see red, so red letters may be a poor choice.
Charlottesville Area Mountain Bike Club www.cambc.org
No doubt signage play vital roll on the track, they give us directions,and other safety instructions. I always capture the photos of important sign, which comes into my track.