2179/120 means each one has to cover about 18 miles. Assuming they average 36 mph, assuming each trunk road is 2 lanes in both directions (assuming one pass per lane and not including time to clean up the shoulder/median), and assuming they spend 50% of their time gritting/plowing new roads rather than filling up/changing drivers/driving on plowed roads to get to or from their target road, that's one pass every four hours. Seems quite reasonable (if a bit undersized) to me as a Michigander.

We're having unseasonably warm weather here - it's 45 and sunny instead of a typical 25 and overcast - so all the snow has melted in Southwest Michigan and our state plow tracking map is basically blank today:

https://mdotjboss.state.mi.us/MiDrive/map

We have about 800 snowplows to cover 12,000 miles of state/trunk roads (15 miles per vehicle). It requires about 2,000 operators to keep those 800 plow trucks running 24x7 when it's really snowing.