Geelhart, Chris; Grier, Tom; Morrison, Jeff; Mortell, Robert; Sanderson, Dale; Summa, Michael (2023). "End of US highway 18". US Ends.com. Archived from the original on December 22, 2016. Retrieved October 6, 2014.
web.archive.org
Geelhart, Chris; Grier, Tom; Morrison, Jeff; Mortell, Robert; Sanderson, Dale; Summa, Michael (2023). "End of US highway 18". US Ends.com. Archived from the original on December 22, 2016. Retrieved October 6, 2014.
"State Highway Data Viewer". South Dakota Department of Transportation. Archived from the original on November 4, 2023. Retrieved January 7, 2024.