The National Parks and Federal Recreational Lands Access Pass
The Access Pass is a free lifetime access pass for any person with a disability or for the caregivers of a person with a disability. It allows access to ALL national parks for free.
All that is required is a letter from the physician stating the disability and you take it to your local National Forestry & Wildlife Division to get the pass. Each pass will admit up to 4 adults and children under 16 are always free.
More information about the Access Pass at the USGS web site.
