The National Park Service (NPS), entrusted by the public to preserve and protect the nation's significant natural, cultural, and historic resources, is also mandated to provide for the public's enjoyment of these (National Park Service Organic Act, 16 U.S.C.1.). Since the first electronic report of visitation statistics in 1979, the number of recreation visits to national park units has increased from approximately 205 million to 279 million in 2011. Some parks, such as Zion National Park, experienced such high visitation during peak season that in 1997 a shuttle bus system was implemented as...