While it's crowded in the first few miles, it does thin out. Streets are wide enough and the spectators will cheer you on. Chicago has seeded start corrals for the fastest 6000 runners. Based on your previous half or full times in the past two years, you may qualify for one of the four corrals in front of the open field (see their website). Being in one of those corrals puts you with runners with similar abilities. You can also run with a pace group. PRs are easy there; the weather may be the real factor (it has been warm the past few years)—show up early and stay calm. After a little dodging and weaving in the first couple of miles, things do thin out. I've run it 13 times, on my way to #14 this year—and have PR'd there many times. Good luck!