first national parks to adopt an alternative transportation system by initiating a shuttle bus system in the Bear Lake Road corridor in 1978 that continues to operate during the peak visitor use season (1). In 1999, RMNP initiated a transportation study to assess existing visitor use, transportation-related problems, and potential solutions (Parsons, Brinckerhoff, Quade, and Douglas, 2000, Rocky Mountain National Park Transportation Study, unpublished report.). This study concluded that the shortage of parking spaces to meet visitor demand was the most significant transportation problem in the park. The study found that about 46% of summer visitors who would like to park at certain trailheads could not do so legally. Furthermore, the study findings suggested that when parking lots were full, visitors often parked illegally in spaces designated for disabled visitors, on road shoulders, or on alpine tundra, which results in safety concerns and resource damage. In 2001, to address the issue of parking lot shortages and related effects on visitors’ experiences and park resources while, at the same time, accommodating growing numbers of park visitors, RMNP implemented an expanded 10-vehicle shuttle bus service from the main shuttle parking lot off Bear Lake Road (referred to in this paper as the “Bear Lake park-and-ride lot”). The shuttle operates from early June through early October and provides service to the Bear Lake, Glacier Gorge, and Fern Lake trailheads and several points in between. Before 2001, approximately 156,000 people rode the Bear Lake and Fern Lake shuttles annually. Transit service has improved every year since then; in 2006, ridership had increased to around 270,000 passengers. With an increasing percentage of visitors accessing trailheads in the Bear Lake Road corridor via the shuttle bus rather than in pri- vate vehicles, the constraint to visitor use levels associated with trailhead parking lot capacities has been effectively eliminated. Thus, while the park’s shuttle service has helped to alleviate park- ing pressure at popular trailheads in the Bear Lake Road corridor, it may also be enabling levels of visitation that cause visitor crowding and resource impacts. This issue is potentially exacer- bated by the park’s shuttle service having been operated, to date, according to visitor demand; as the number of visitors waiting at the Bear Lake park-and-ride lot to board shuttle buses increases, the number of buses operating within the system is increased until there are no more buses available. This approach is designed to reduce waiting times at shuttle bus stops and onboard crowding, and it potentially increases the convenience of using the shuttle service. However, the effects of “demand-driven” shuttle service Modeling the Effects of Shuttle Service on Transportation System Performance and Quality of Visitor Experience in Rocky Mountain National Park Steve Lawson, Robert Chamberlin, Janet Choi, Ben Swanson, Brett Kiser, Peter Newman, Chris Monz, David Pettebone, and Larry Gamble 97 Rocky Mountain National Park (RMNP) was one of the first national parks to adopt an alternative transportation system: a shuttle bus system initiated in 1978. To address parking lot shortages while accommodating growing numbers of park visitors, RMNP expanded its shuttle bus ser- vice in 2001. Although the expanded shuttle service has helped to allevi- ate parking congestion at popular trailheads, expansion may also be enabling levels of visitation that cause or exacerbate visitor crowding. Thus, there is a need to evaluate and potentially refine RMNP’s shuttle service according to the amount of visitor use that can be accommodated at popular destinations in the park without unacceptable effects on the quality of visitors’ experiences. This study evaluated and quantified transportation system performance and visitor crowding at popular recreation sites in the Bear Lake Road corridor resulting from RMNP’s shuttle service operations. The study used integrated transportation and visitor use modeling to provide quantitative estimates of the extent of parking congestion, transportation-related greenhouse gas emissions, transit operating costs per passenger, and visitor crowding associated with existing and alternative transit service operations scenarios. The National Park Service will use information from the study to refine the operation of shuttle service in RMNP in a manner that both optimizes transportation system performance and protects the quality of visitors’ experiences. Further, the study framework can be generalized to other public lands units to design and operate transit service in accordance with transportation, resource, and visitor experience objectives. Increasingly, the National Park Service (NPS) is relying on alterna- tive transportation systems to provide visitors access to the national parks in a manner that potentially reduces traffic congestion, enhances visitors’ experiences, and more effectively protects park resources. Rocky Mountain National Park (RMNP) was one of the S. Lawson, R. Chamberlin, J. Choi, B. Swanson, and B. Kiser, Resource Systems Group, Inc., 55 Railroad Row, White River Junction, VT 05001. P. Newman, Warner College of Natural Resources, Colorado State University, 1401 Campus Delivery, Fort Collins, CO 80523-1401. C. Monz, Ecology Center, Utah State University, 5205 Old Main Hill, Logan, UT 84322-5205. D. Pettebone, Yosemite National Park, El Portal, CA 95318. L. Gamble, Rocky Mountain National Park, Estes Park, CO 80511. Corresponding author: S. Lawson, slawson@rsginc.com. Transportation Research Record: Journal of the Transportation Research Board, No. 2244, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 97–106. DOI: 10.3141/2244-13