@Brandon9000,
People usually actually go 5-7 miles over the speed limit (in my experience in MN, WI, CA, IL, and OH, but that may not be universal). So if the posted speed limit is 55, and people are actually driving 60-62 mph, and someone's going 45, that's a significant disruption.
The disruption to traffic flow -- especially if you're not in the right lane (as in, if it's a single-lane freeway, or if it's multi-lane and you're driving in the fast lane) can result in a ticket.