> I remember in Chicago the interstates having posted speed limits of 45mph... the average flow of traffic outside of rush hour was easily north of 70mph.

This comment seems a bit odd to me. I Google about it and learned (from various sources):

    > 45 mph (72 km/h) in downtown Chicago, where all the major interstates merge

This excludes construction or work zones.

That seems pretty reasonable. I've seen a few places in the US where several major interstates merge and the post speed limit is quite low -- 45-55 mph.