Talk to your local emergency management dept.
What we have always done is just worked with the local EMS, one of their staff is usually a "emergency surveyer" they will come out to your site and inventory the critical response paths. As a "best practices that I have seen here are a few things that really stuck ou to me. Have a really good trail map. Appropriately and acurately sign the trails. Use intersections as meeting and evacuation points, try to get all intersections GPSed. Communicate good emergency practices through signs, your club and a bike patrol. Have the city assign street addresses to the individual trail heads and ensure they are in the 911 data base. I think that building a trail around evacuating the less than 10% of users that may get hurt is selling the trail experience short. Use common sense and try to just think about evacuating folks as easily as possible. Remember most EMS departments are very well trained individuals and most seem to really enjoy backcountry rescues.
Good luck,
Ben