Our research team at the University of Southern California, Center for Risk and Economic Analysis of Terrorism Events (CREATE, a US Department of Homeland Security Center of Excellence), was commissioned in 2007 to explore the question of why, and to what extent, people change their travel choices following terrorist attacks. This case study focuses on one of the articles in this project—‘Exploring Reductions in London Underground Passenger Journeys Following the July 2005 Bombings’—which was published in the journal Risk Analysis. This case study first highlights the challenges of estimating the relevant counterfactual: what would London Underground passenger journeys be if the terrorist attack not occurred? The second challenge involved explaining passenger journey reductions; our approach was to explore alternative explanations—such as preexisting trends, seasonal changes, economic growth, special events, and station closures following the attack—for the reduction in passenger journeys following the attacks. We were unable to either rule out or confirm due to a lack of data the common sense explanation that people were more fearful of using the London Underground in the aftermath of the attacks.