Since the September 11 attacks, policymakers are paying increasing attention to the adequacy of the U.S. trauma care system to handle potential mass casualty incidents. This attention has led to questions about how well the trauma system covers the population for day-to-day trauma, such as motor vehicle accidents and gunshot wounds. Although the number of trauma centers has increased in the last decade, no national plan exists to assure that everyone has timely access to a specialized trauma center if needed. This Issue Brief summarizes a new study that estimates the proportion of residents that can reach a trauma center by ground or air ambulance within one hour of where they live, using objective measures of travel times and distances.