The U.S. transportation system-which is composed of multiple interconnected modes of transportation, including aviation, freight and passenger rail, highway, public transit, and pipelines-is an open, accessible, and complex system that moves billions of passengers and millions of tons of goods each year. According to the Department of Homeland Security (DHS) Transportation Systems Sector-Specific Plan, the system's size and importance to the country's safety, security, and economic well-being make it an attractive target for terrorists.