Before I make major highway reclassification edits, I research what routers recommend, specifically Bing since the default OSM routers are heavily influenced by OSM data itself. Bing suggests that taking SR 146 is a few minutes shorter than going down to I 70. 

AADT data (https://odot.public.ms2soft.com/TDMS.UI_Core/trafficviewer) also suggests that SR 146 at SR 16 is a very major fork, with nearly half of the traffic (~7,000 AADT) heading east along the freeway exiting off or merging onto it westbound at the interchange with 146 coming to or from Zanesville, with the other ~9,000 AADT continuing east along SR 16. To me it seems that each route is about equally viable, considering both the routing suggestions and traffic data.

Regarding functional classification and NHS designations - functional classification is inconsistent by state and really only exists to be used by the state DOTs mainly to identify how roads should be funded and isn't very useful for OpenStreetMap. As for the National Highway System, it includes various types of roads and highways as well as excluding some major corridors, and is primarily for strategic mobility rather than general civil navigation.