oh, for what it's worth, I apologize a bit to you. I know I'm likely coming off as a jerk, but I assure you it's unintended. You're just out for a bike ride.
I hadn't heard of Slow Roll. They might be better at handling traffic than the one Critical Mass ride I saw. They had several thousand riders going through a roundabout. It looked spontaneous as they had no detours posted, and a few cars were stuck in the loop for about an hour. They ride at the same time each (week? month?) so I just remember to not drive through that part of downtown on Fridays in the summer.
Really, though, I've looked into similar things, and can't understand how they're able to do it next to a police precinct without posting detours and without being fined or traffic controlled. But, it's Detroit. There are probably 10 cops covering downtown at any time, and they might not have the resources to police traffic on a Friday evening.
That said, I'm actually interested in this. Did you guys have to go through city hall and set up the detour routes and get permission from the affected businesses to lay down your route? The city I tried to set something up in would have allowed it, but there was a lot of paperwork and it was smaller than an organized ride.
Are you one of those critical mass riders who shuts down Campus Martius for a few hours a month?
'Cause if you guys could stop and let traffic through on our way home from work, that'd be really helpful. You could pass out pamphlets and get more riders that way.