Mountain bikers say they ride off-road because it's safer than riding on the road. Is that true? No: "In some Canadian provinces, mountain biking is a popular sport that involves riding on steep slopes, through densely wooded trails, and jumping obstacles and cliffs. It involves considerably higher injury risk than transport cycling." (http://bmjopen.bmj.com/content/5/11/...full.pdf+html)
. In addition, road- and off-road biking have different goals: off-road biking doesn't generally provide transportation from one point to another, so it can't substitute for transportation cycling.
The State of California considers mountain biking a "hazardous recreational activity":
"'hazardous recreational activity' means a recreational activity conducted on property of a public entity that creates a substantial, as distinguished from a minor, trivial, or insignificant, risk of injury to a participant or a spectator." Notice that bicycling or driving on pavement is not considered a "hazardous recreational activity".