There is a lot of traffic on the Hwy during the daytime in mid-summer, but it's rarely bothersome. Most of the time you don't even notice it - or at least I don't. (I stay there from mid-May to early October). The only exception is before and after Sturgis in July, when there are pleny of Harleys on the Hwy.

However, since much of the summer traffic is tourists, the majority are off the road by 6:00pm or so, and practically no one is on the road after dark and it's quiet as a mouse all night (maybe just a slurping sound or two from feeding browns in the river). Plus, there's more deer on the road than vehicles after dark - another reason not to drive at night.