In Marin, equestrians are allowed on 88% of the singletrack. There are no environmental reasons why bike access should be treated any differently.
One of the most frequently cited studies of soil erosion...found that foot- and hoof-powered activities (hiking and horseback riding) had a greater erosive potential than did wheeled activities (off-road vehicles and mountain bikes.) A similar experiment was conducted in a Provincial Park in southern Ontario, producing comparable results...and found no significant difference in the effects on soils of the two activities. A study that was conducted on a multi-use trail network in Kentucky and Tennessee found that of all types of trails, bike trails were found to be the narrowest, to have the least amount of soil loss, and to have the least incidence of running water on the trails —Marion & Olive 2006, Quinn & Chernoff, 2010
The vast majority of studies conducted on various impacts on trails, flora, and fauna have demonstrated that by nearly every measure, mountain bikes have the same or significantly less impact than hiking and equestrian activities.
Many communities around the world have incorporated this knowledge into their land management practices and now have diverse trail networks which include sustainable access for off-road bicyclists.
For more information, please view our Mountain Biking in Marin Fact Book.