Oftentimes comically lower. 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.

Looking even at normal arterial streets, many streets in Seattle are marked 25, but you'd be hard-pressed to find even a cop going under 30 most of the time.

I truly don't understand US road design. The construction of the road and the posted speed limit almost never are even gently correlated other than on a few select residential side streets in a few select cities who have rebuilt streets based on safety studies.