Earlier quoted context omitted.
I drive those highways every day, that's absolutely uncommon unless you're in the exurbs. I drove to Aurora yesterday and speed of traffic was barely over 70. If you're cruising at 90 in cook county you're asking for a court date. 15mph over the limit mandates it, and I think the highest speed limit is 60 on 290 out in Arlington Heights. It's 45 on the Kennedy once you get into the city right now.
Interesting - I used to drive I-88 daily from Wheaton to Aurora. Although I was going counterflow, I rarely saw the speed of other vehicles average less than 80-85.
I think it's almost impossible to judge the speed of others unless you're consciously trying to maintain speed with the flow of traffic. If you're constantly passing people as you travel you're not doing that.