^Federal Highway Administration. Section 2D.11 Design of Route Signs. Manual on Uniform Traffic Control Devices 2009 2nd revised. Washington, DC: Federal Highway Administration. 2009 [2023-02-25]. (原始内容存档于2023-05-31). Guidance: State Route signs...should be rectangular and should be approximately the same size as the U.S. Route sign....The shape of the white area should be circular in the absence of any determination to the contrary by the individual State concerned.
^County Roads. Hampshire Council of Governments. [2017-08-16]. (原始内容存档于2017-08-17) (英语).