^^ Don't forget that we have a wide variation in climate.
So although 100KMH looks too slow on a nice summer day it's probably too high in a snow storm at 3am. Rather than go to the expense of having signs that change the speed limit with conditions, everybody seems to understand that, if all is well, you can safely travel 120KMH on the 400 series of highways without getting tagged.
I think it works fine.
We have the technology to have variable speed limits, tailored to conditions. The method of posting already exists on 400 series highways, in the GTA.