speed limits

There's this interesting thing that happens on the main thoroughfares where I live; on the biggest artery of the city, people tend to drive ten to fifteen miles over the speed-limit, with a few cutting people off and weaving in and out of traffic. But there's this other road, with the same speed-limit, where people usually drive five to ten miles under. That is, until the limit drops ten mph, then they decide to treat it like it's the original limit, and drive five to ten over, which is actually now fifteen to twenty over. I don't understand it.