National Park Service - Visitors statistics

Reference 1: Visits.

Reference 2: Blue Ridge Parkway, Golden Gate National Recreation Area, Great Smoky Mountains National Park, Lincoln Memorial, Lake Mead National Recreation Area, George Washington Memorial Parkway, Gateway National Recreation Area, Natchez Trace Parkway, Vietnam Veterans Memorial and Grand Canyon National Park.

Source: http://en.wikipedia.org/wiki/National_Park_Service