Is there a term for...

[cpzilliacus]

...a US highway that closely follows an interstate that has hardly any traffic on it but never gets decommissioned?  Thinking of roads in Oklahoma for example, or US 90 through east Texas. I can't think of a term that fits

U.S. 40 is the longest highway in Maryland at over 200 miles.

But it cannot make up its mind if it wants to be on one of the Interstates or "by itself."

It is multiplexed with I-68 for a long run across the mountains of Western Maryland (including the substandard Cumberland Thruway), but after a short distance with I-70 at the narrowest part of the state near Hancock, it sets off on its own, following an old alignment through Hagerstown and to Frederick.  At Frederick, it rejoins I-70 until it reaches the western Baltimore suburbs, where it diverges from I-70 and crosses Baltimore City, picking up the decommissioned I-170 and then Orleans Street and then onto Pulaski Highway.  As Pulaski Highway, it runs parallel to I-95 all the way to the Delaware border, but it has no multiplexed section with I-95.