Update 12/19/23
The detestable “Share the Road” sign has been eliminated and the “Bicycles May Use Full Lane” sign has been replaced with a “Bicycles Allowed Use of Full Lane” sign by the US Department of Transportation in the most recent edition of their manual for traffic control devices, published today. States must adopt these new standards within two years.
This sign may be used on roadways where no bicycle lanes or adjacent shoulders usable by bicycles are present and where travel lanes are too narrow for bicycles and motor vehicles to operate side-by-side. (Which, by the way, is true for the travel lanes of most roads). See Section 9B.14. https://mutcd.fhwa.dot.gov