160 1980s, much was accomplished to develop an understanding of avalanches, and methods of forecasting and active control measures using explosives or artillery were developed and refined. In the mid- 1980s the USFS ceased its involvement in avalanche research and, since that time, there has been no national organization coordinating avalanche work. Today, regional avalanche centers funded by state governments, and to a lesser extent by the federal government, are responsible for hazard forecasting and some active control activities for recreational areas as well as along transportation routes. Avalanche risk reduction measures employed by state departments of transportation focus almost exclu- sively on artificial release via detonation of explosives in the starting zone coupled with road closures during explosive control and to clear any resulting avalanche debris from the roadway. Although this approach provides for improved safety to the traveling public, the use of explosives can be problematic in both populated areas and where critical wildlife habitat exists. Moreover, active control is not always 100% effective. During extreme storm periods, avalanche control per- sonnel may not have access to the starting zone, and hazard mitigation can only be accomplished by closing the road altogether. This is often an extremely costly option since shutting down transportation lines interrupts commerce and prevents access to mountain regions whose economies are dependent on tourism. In Colorado, for example, I-70 carries traffic from the Denver metro region through the mountain resort regions, where it has been estimated that tourism-related transactions contribute $800,000 per hour to the local economy (1). On December 31, 2007 (New Year’s Eve), a 60-mi stretch of I-70 west of Denver was shut down for almost 24 h because of extreme avalanche danger, stranding 2,000 people, who were forced to stay in Red Cross shelters overnight. Undoubtedly this served not only as a major inconvenience but also negatively affected the tourist industry. In Washington State, I-90 serves as the major transportation link between Seattle and Spokane and carries 32,000 vehicles per day between the western and eastern portions of the state. I-90 passes through several areas of the Cascade Range that experience regular avalanche activity and, to keep it open year-round, the Washington State Department of Transportation (DOT) has developed a compre- hensive program to mitigate the avalanche danger. This program relies heavily on forecasting and active control to reduce avalanche hazard, although some passive defense measures are utilized. Epito- mizing the potential for high economic impacts of avalanche hazard is the winter of 1996 to 1997, which saw 276 h of closure of I-90 at Snoqualmie Pass, with an estimated cost to the state’s economy of roughly $144 million (2002 U.S. dollars) (2). The long-term costs (direct and user delay related) of active control to mitigate avalanche hazard can be staggering and in some cases far outweigh the initial costs of structural passive avalanche defense. Implementation of Structural Control Measures to Mitigate Avalanche Hazard Along Transportation Corridors Joshua T. Hewes, Rand Decker, Scott Merry, and Jamie Yount Avalanche hazards along transportation corridors in the United States have traditionally been addressed by forecasting their potential and actively controlling them through explosive release while the roadway is closed. This approach reduces the threat of avalanches cascading onto the roadway and thus reduces danger to the traveling public. However, active control methods cannot always be implemented in a timely fash- ion, can be ineffective, and can have large associated economic impacts. An alternative to active control is passive, structural avalanche defenses. They are passive in that they do not require maintenance during winter storm periods. Structural defense measures include snow sails, snow- supporting structures, and snow sheds. Despite their extensive use in Europe and their potential for effectively reducing avalanche hazards, few examples are found in the United States. The potential for negative impacts to the visual attributes of the landscape has been a significant reason for their lack of domestic use. This paper discusses several types of structural defense measures, criteria for their selection at a given site, and their relative effectiveness. Passive structural defense measures designed for implementation at the Milepost 151 avalanche on US Route 89/191 near Jackson, Wyoming, are described. Details are given on important collaborations between landscape architects and engineers that led to successfully addressing National Environmental Policy Act require- ments for retention of visual attributes at the Milepost 151 avalanche site in the presence of snow support structures deployed for the purpose of avalanche hazard reduction. The mountainous regions of the western United States have experi- enced dramatic growth in the past several decades, and with this development has come an increased risk of motorist injury or death due to avalanches impinging on roadways. Although this risk is increasing, there has been relatively little new research conducted on avalanche hazard mitigation techniques. Starting in the 1950s, the U.S. Forest Service (USFS) served as the leader in avalanche forecasting, control, research, and education. This role resulted from the development of downhill skiing areas located on USFS land, and avalanche study centers were established at Alta, Utah, and in Fort Collins, Colorado. During the period between the 1950s and early J. T. Hewes and R. Decker, Department of Civil and Environmental Engineering, Northern Arizona University, P.O. Box 15600, Flagstaff, AZ 86011. S. Merry, c/o Rand Decker, Kleinfelder, Inc., P.O. Box 15600, Flagstaff, AZ 86011. J. Yount, Wyoming Department of Transportation, P.O. Box 14700, Jackson, WY 83002. Corresponding author: J. T. Hewes, Joshua.Hewes@nau.edu. Transportation Research Record: Journal of the Transportation Research Board, No. 2169, Transportation Research Board of the National Academies, Washington, D.C., 2010, pp. 160–167. DOI: 10.3141/2169-17