In the context of homeland security, critical infrastructures are "those physical and information technology facilities, networks, services and assets which, if disrupted or destroyed, would have a serious impact on the health, safety, security, or economic well-being of citizens or the effective functioning of governments." Transportation infrastructures are a key component of a nation's critical infrastructures, covering physical assets such as airports, ports, and railway and mass transit networks as well as software systems such as traffic control systems. In effect, among various critical infrastructures spanning a range of economic sectors and government operations, transportation is widely viewed as one of the most significant and impactful. A 2002 study concerning the significance of infrastructure components and the consequences of a destructive event rated transportation as "extremely significant." Other components at this highest level of significance were communications, power, emergency response personnel and assets, and national security resources.