Washington has a system of state highways, called State Routes, as well as an extensive ferry system which is the largest in the nation[80] and the third largest in the world. There are 140 public airfields in Washington, including 16 state airports owned by the Washington State Department of Transportation. Seattle-Tacoma International Airport (Sea-Tac) is the major commercial airport of greater Seattle.[81] Boeing Field in Seattle is one of the busiest primary non-hub airports in the US.[82] The unique geography of Washington creates exceptional transportation challenges.