Looking at some intersections in Google Street View shows the green blade of a street name, not the square beehive of a numbered highway.  This leads me to believe that yes, this is a street named "Old Highway 91", and as such, putting the name into a "ref=" tag would be incorrect.

This is a very common pattern in the western United States: a highway or highway routing gets decommissioned and turned into an ordinary road with a name of "Old Whatever Highway".