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The Hajj, the annual Muslim pilgrimage to Makkah in Saudi Arabia, is one of the largest pedestrian events in the world. Each year, up to four million pilgrims approach the holy sites in the region of Makkah to perform their religious duty. The key ritual, the stoning-of-the-devil, is particularly crowded. Until 2006, several crowd-related disasters led to thousands of casualties. In the aftermath of such a disaster in early 2006, the Ministry of Municipal and Rural Affairs of the Kingdom of Saudi Arabia (MOMRA) launched many projects to prevent future crowd-related accidents. In particular, MOMRA began the development of an operations research (OR)-based decision support system (ORDSS) for crowd management. ORDSS employs a range of tools from OR, analytics, and crowd dynamics. At its core, it implements a scheduling tool and a real-time video tracking system. The video tracking system measures infrastructure utilization, and an integrated series of mixed-integer programs and quadratic programs balance capacity utilization by considering preferred stoning times and infrastructure capacities. The ORDSS provides MOMRA with solutions that enable uncongested and smooth pilgrim flows and extensive real-time reporting. From 2007 to 2014, OR helped stop the tragic loss of human life that resulted from these crowd-related accidents. Unfortunately, a crowd-related disaster, which resulted in hundreds of casualties, occurred during Hajj season 2015; however, for this Hajj season, the authors and MOMRA were no longer in charge of the scheduling and routing recommendations for the stoning-of-the-devil ritual during the Hajj.

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