Over here in central Ohio, I've seen both happen. We had a stretch that went from 35 to 25 mph, and people slowed from 40 to 32 (that's how fast I drive to stay with the flow there). Then I remember when the limits were raised way back from 55 to 65 on the freeways. We all went 65, despite many newspaper blurbs about the police stating they'll nab drivers going 66.

Also, I like driving in Michigan much more than in Ohio. That extra 5 mph seems to do very well, and traffic is almost always smooth and safe up there.