University of Texas M. D. Anderson Cancer Center in Houston is a highly regarded academic institution that centers around cancer patient care, research, education and prevention. San Antonio's South Texas Medical Center facilities rank sixth in clinical medicine research impact in the United States. The University of Texas Health Science Center is another highly ranked research and educational institution in San Antonio. Both the American Heart Association and the University of Texas Southwestern Medical Center call Dallas home. The institution's medical school employs the most medical school Nobel laureates in the world. Transportation Texans have historically had difficulties traversing Texas due to the state's large size and rough terrain. Texas has compensated by building America's largest highway and railway systems. The regulatory authority, the Texas Department of Transportation (TxDOT), maintains the state's immense highway system, regulates aviation, and public transportation systems. The state is an important transportation hub. From the Dallas/Fort Worth area, trucks can reach 93 percent of the nation's population within 48 hours, and 37 percent within 24 hours. Texas has 33 foreign trade zones (FTZ), the most in the nation. In 2004, a combined total of $298 billion of goods passed through Texas FTZs. Highways The first Texas freeway was the Gulf Freeway opened in 1948 in Houston. As of 2005, of public highway crisscrossed Texas (up from in 1984). To fund recent growth in the state highways, Texas has 17 toll roads (see list) with several additional tollways proposed. In Central Texas, the southern section of the State Highway 130 toll road has a speed limit of , the highest in the nation. All federal and state highways in Texas are paved. Airports Texas has 730 airports, second-most of any state in the nation. Largest in Texas by size and passengers served, Dallas/Fort Worth International Airport (DFW) is the second-largest by area in the United States, and fourth in the world with . In traffic, DFW airport is the busiest in the state, the fourth busiest in the United States, and sixth worldwide. American Airlines Group's American / American Eagle, the world's largest airline in total passengers-miles transported and passenger fleet size, uses DFW as its largest and main hub. It ranks as the largest airline in the United States by number of passengers carried domestically per year and the largest airline in the world by number of passengers carried. Southwest Airlines, headquartered in Dallas, has