I-696, aka the Detroit autobahn. Go less than 85 and you will get passed with extreme prejudice.
CC
Instead of a billion or so on I-75, I would rather see Mound road upgraded to a freeway connected to the Van Dyke freeway, along with the Davidson finished. Imigane 1-96 connected all the way to Utica and beyond. This would take a load off of I-75 and I-696.