94 probably sees as much truck traffic as it does in part because i-80 is toll. Put a toll on 94, you'd probably reduce traffic because cross country routes would lose any motivation to ever come north. That would not be a bad thing in most people's view I think! The absence of toll roads in MI has a long (and probably now obsolete) history. Of course the auto-plants didn't want them, and that is probably the biggest reason, but back in the 40's 50's and 60's, when the road nets were being planned but railroads were still a thing, there was an arg that since MI was a peninsula, we had poorer cross state rail connections than other states so it was only 'fair' that we not have toll roads since there were fewer shipping/travel alternatives. That seems a quaint arg today, but I remember it being a pretty stock political talking point when I was young - so that is a little bit of how we got to where we are.