It often is... but on the high side.  Not sure about the south end, but I-25 north of Denver really shouldn't have the 75 MPH speed limit on it that it does. There's WAY too much traffic for that speed, IMO.  It was fine with less traffic, but there's near constant compression points along the way now where you're doing 75 then have to slam on the brakes.  It's just not a very safe road at that speed with this much traffic.

IMO, they need to lower it to 70 or even 65.  Or better yet, install traffic sensors and electronic speed limit signs and employ variable speed limits.  When traffic is light, use a 75 MPH speed limit; when it's heavy drop it to 65.