On Thursday, August 8, 2013 3:18:53 AM UTC-7, I love Mike wrote:
http://atfiles.org/files/pdf/Mountai...west-white.pdf





Key conclusion:



The findings from this study reinforce results from previous research that certain impacts to mountain bike trails, especially width, are compa- rable or less than hiking or multiple-use trails, and significantly less than impacts to equestrian or off-highway vehicle trails.



Note:



Data for the study were collected from 319 sample points gathered from 162.3 miles of mountain bike trails in five common ecological regions of the southwest United States.

Junk science. Not worth the paper it's printed on. Not that you would know the difference.