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I did some digging, and found an [ADA guidelines document](https://www.americantrails.org/images/documents/TN-trail-ada.pdf) from the Tennessee Department of Environment and Conservation. Look at page 3, which details all the technical requirements. It is from 2007, so take it with a pinch of salt.

Bear in mind always that these are _guidelines_, not code or law. Basically anyone can label their trail accessible if they think it is, and that's perfectly legal.

 * Trails must be at least 36" wide.
 * Boardwalks or similar must not have gaps wider than ½".
 * Obstacles must not be more than 2" high (3" when slope is <5%; 1" when slope is >5%).
 * Running slope: not greater than 5% for long runs (shorter runs of steeper slope are permitted; see the linked document). Not greater than 14% for any distance.
 * Cross slope: not greater than 2% for long runs (shorter runs of steeper slope are likewise permitted). Not greater than 12% for any distance.
 * Cross slope >5% is not permitted when running slope is >5%, and vice versa.