Natzke, Stefan; Neathery, Mike; Adderly, Kevin. What is the National Highway System?. National Highway System. Federal Highway Administration. 2012-09-26 [2013-01-15]. (原始内容存档于2012-08-28).
Natzke, Stefan; Neathery, Mike; Adderly, Kevin. What is the National Highway System?. National Highway System. Federal Highway Administration. 2012-09-26 [2013-01-15]. (原始内容存档于2012-08-28).