I don't know about copyright, but "One Less Car" is a registered trademark with the US Patent and Trademark Office.

http://tess2.uspto.gov/bin/showfield...ate=7om76g.2.1

Seems to be registered to Locomotion in Favor of the Earth, Inc., a consulting company located in Maryland. Curiously, and perhaps coincidentally, onelesscar.org is also located in Maryland.

From the Trademark listing:
"Consulting services, namely, providing consultations to others in the field of bicycling and pedestrian safety; lobbying services, namely, advocating for the interests of bicyclists and pedestrians."

I suspect you would have difficulty in keeping the rights to this name considering that there is an active trademark related to alternative transportation. However, you would be best served to consult with a patent attorney to know what your position really is.