I travel from Chicago to Detroit regularly in my MS. I've hit all the SCs on I-94/96 (St. Joe, Marshall, Ann Arbor, Livonia). Historically, they are never full. The last few trips, however, I've noticed that traffic is really picking up with lots of M3s. I've been lucky and pulled in just as a slot was opening, so no wait times, but I'd be interested to hear other's experiences. Have you had to wait for a SC slot on I-94?