Map of US highways 16 and 20 and families
US 20 is represented by the purple line running from Newport OR to Boston MA. Alternate routes in New York and Ohio are shown in blue, and historic routings are shown in red. US routes are not signed through Yellowstone National Park, so I've indicated the implied route of US 20 with a dashed line.
US 120 (historic) is the red line from Philadelphia to Erie PA.
US 220's original route is shown on this map (the blue line from Sayre PA to Cumberland MD). As the designation was extended further south from Cumberland, the highway began to resemble something other than a branch of US 20, and it should've been renumbered. I think US x15 would've been the best option; you can view a map showing US 220's entire route on my US 15 page.
US 320 (historic) is the red line from Riverton to Shoshoni WY.
US 420 (historic) is the red line from Cody to Deaver WY.
US 16 currently runs from Rapid City SD to Worland WY, then joins with US 20 to the east entrance of Yellowstone NP (purple line). Historically US 16 followed a different route between Rapid City and Moorcroft WY (red line), and it also continued west of Rapid City all the way to Detroit (including a ferry segment across Lake Michigan).
US 116 (historic) is the red line from from Sheridan to Ucross WY.
US 216 (historic) essentially ran along what is now US 16 between Moorcroft and Rapid City, back when the US 16 designation was assigned along the north edge of the Black Hills.