They have long lines coming back into the US form Canada, and it sits on the bridge. It looked like the trek may by a delay of 1 hour to get through US Customs. Going to Canada I do not believe is as strict. I-75 dead ends her and if you venture too far, you are caught up on the bridge, and may have to pay a toll to get back or turn around.
I was staying at a hotel in Sault Ste. Marie and had a free shuttle arranged to pick me up and bring me to the hotel. A shuttle arrived, which the driver verified was the hotel shuttle. He took a ridiculous amount of time to leave (he was apparently waiting for packages) and took a leisurely drive back to the hotel, even though he knew I was driving into a snowstorm on my way home. Upon my arrival, I gave what I felt to be a generous tip. The driver responded in a calculating voice "That's not enough." I was really taken aback, but apparently it was a paid shuttle service, and the guy never mentionned a rate and I had asked specifically if he was the hotel shuttle. There were no rates listed in the vehicle or any indication that it was not the hotel shuttle as he had said. Apparently this particular individual pulls this scam with people very frequently, so beware...