Totally agree. I believe that people drive at their comfort level regardless of conditions. Of course there are those that are either in a hurry for whatever reason as well as those who are nervous driving anywhere near the posted limits. And there are those who are oblivious to adverse conditions due to weather or heavy traffic. IMO, speeds on freeways should be enforced at + or - 10 whatever the limit is. Keep the +10 on secondary highways and go with - 20 to accommodate those not comfortable at higher freeway speeds. No stats, but I believe that the consequences of driving 20 under can be more serious than driving 20 over.
For me, I tend to drive 10 over with truck only and 10 under when pulling my trailer.