Here are resources about what constitutes an accessible trail, as well as information about general trail rating.
If you have additional resources, please contact me using the form in the sidebar.
Access Recreation Here is a wonderful project in Washington and Oregon.
“Access Recreation [AR] is a Portland, Oregon ad hoc committee that has been developing uniform guidelines for minimum information that should be provided about hiking trails and outdoor recreational facilities, that would benefit hikers with disabilities. These guidelines can be applied to websites, printed materials and at trail sites. …..When put into place, these guidelines will provide the public with easy access to better information on the accessibility of recreational and hiking trails in the states of Oregon and Washington and nationwide.” Georgena Moran, founder and project coordinator, says it is a “two-year project [ends June 2016] and as for now only two trails have been trail mapped the with video accompaniment.” Article by Georgena, “Trails for Everyone”.
Lots of links for how to make an area/trail ADA compliant on the American Trails website.
International Mountain Bicycling Association’s rating system for trail difficulty.
The National Center for Accessibility at Indiana University.
An excellent article by San Francisco Bay area Bonnie Lewkowicz : Opening the Door to Nature for People with Disabilities which was written in 2006 and is still relevant.
Thoughts on rating trail difficulty
US Access Board guidelines for outdoor recreation. Scroll down to 1017 for particulars regarding Trails.
US Dept. of Transportation Federal Highway Administration. Chapter 5 Trail Design for Access
United States Forest Service Accessibility Guidebook for Outdoor Recreation and Trails