I have tried to put together a system to suggest the difficulty of bicycling the roads in the national parks. Bicycles are frequently only allowed on roads (not trails), but check with the park as some have a few bike-friendly trails and others do not allow bicycles on roads or have special guidelines for cyclists. (For example, Death Valley National Park requires cyclists to bike single file on all paved roads.)
Surface is paved or hard-packed gravel. Average grade is less than 5% and maximum grade is 10%. There should be no technical obstacles to overcome.
Surface may be paved or it may be more natural, but it is firm and stable. Average grade is less than 5% with a maximum grade of 15%. There may be avoidable obstacles, unavoidable obstacles less than 2 inches tall, and 36 inch or wider unavoidable bridges.
Surface is mostly stable but may have some variability. Average grade is 10% or less with a maximum grade that may be more than 15%. Unavoidable obstacles may be up to 8 inches tall and unavoidable bridges may be 24 inches or more. Technical Trail Features (TTFs) may be up to 24 inches with the width of deck at least than half the height.
Surface is widely variable with unavoidable obstacles up to 15 inches, loose rocks, and unavoidable bridges of 24 inches or more. Average grade is up to 15% and maximum grade may be more than 15%. TIFs of 48 inches high with width of deck less than half the height may be present. Short sections may exceed the criteria.
Surface is widely variable and unpredictable. Bikers should expect, at the very least, average grade of at least 20%, max grades more than 15%, unavoidable obstacles up to 15 inches, unavoidable bridges of less than 24 inches, loose rocks, and TTFs of 48 inches or more with the width of deck unpredictable. Conditions may be worse than the criteria in many areas.
Many thanks to IMBA (International Mountain Bicycling Association) for their aid with assigning trail difficulties. Visit the IMBA site here: https://www.imba.com/resource/trail-difficulty-rating-system