Adherence to speed limits seems to vary a lot from state to state in the US. I grew up in Ohio, and you'd better start worrying at 10mph over the limit, very strict pretty much everywhere but notorious on the "Turnpike". Chicago? Total joke, just go! Michigan drivers are pretty quick even on surface streets. Crossing South Dakota the limit was something like 75mph, so I did 80-85 and still got passed pretty frequently. They routinely go 100mph in Texas and don't seem to think much about it.