This information was provided by Highway Commissioner Fred VanKirk, at my request. This was the most recent information provided to the USDOT to certify compliance with the NMSL, hence the data is somewhat old, but still useful.
On rural sections of interstate and interstate-standard highways, the 50th percentile speed was 65 miles per hour and the 85th percentile speed was 72 miles per hour. On other types of "rural expressways" (presumably other types of four-lane divided highways not meeting interstate standards), the 50th percentile speed was 57 miles per hour and the 85th percentile speed was 62 miles per hour Unfortunately, information on the urban interstates in the Charleston area was not available to me because the limit there is 50 mph and, therefore, the information did not have to be submitted to the USDOT. I would estimate on a rogh guess that those roads run at about 65. Also, I estimate the prevailing speed on those other "rural expressways" has risen to about 65 or 66 miles per hour; I do know for sure that it has gone up in the past year.