(Most recently updated 1/11/2020.) Today, Denver is served by seven different US routes: US 6, US 36, US 40, US 85, US 87, US 285, and US 287...
...but when the US highway system was first commissioned in late 1926, only three routes went through Denver. AASHO officials acknowledged the city's importance by placing it at the junction of a major north-south route (US 85, which ran from a Canadian port of entry almost to Mexico) and a transcontinental east-west route (US 40, which connected Atlantic City NJ to the San Francisco Bay Area):
Colorado is somewhat infamous for not signposting its US routes where they are concurrent with interstates. This is more than just signage policy: CDoT route logs actually divide US routes into segments, and these segments are discontinuous in cases where the US route shares pavement with an interstate. So in a sense, CDoT considers US routes to be non-existent wherever their implied route is on an interstate. This notion has resulted in at least one very strange sign, shown here:
Sanderson, May 2017
That was taken looking west on I-270 at its interchange with I-25. Straight ahead on the Boulder Turnpike is the east "beginning" of US 36... that is, assuming one disregards the 1300-mile segment of US 36 behind the camera. (Incidentally, heading the opposite direction, there is a "Begin I-270" assembly, but it is not accompanied with an "End US 36" sign.)