Traffic gridlock in national parks. (National Society for Parks and Recreation).

"The fundamental purpose of the National Park System is resource preservation," according to an article on the National Park Service's Web site. "Visitors can experience and learn about their natural and cultural heritage in parks. Most visitors arrive by private auto and, in some cases, this has begun to threaten the very resources parks were created to protect.

"Congestion in many national parks causes lengthy traffic delays and noise and air pollution that substantially detract from the visitor's experience and overall resource protection," the article continues. "Recognizing that more park roads and more parking lots are not the solution, the introduction of visitor transit systems help to alleviate traffic problems and make parks more accessible. Park roads and visitor transit systems are intended to enhance visitor experience while protecting park resources.

Transportation is, in many ways, the connection between the Park Service's mission of resource protection and visitor enjoyment." For more information, go to www.nps.gov/transportation/alt/index.htm.