They are limits based on ideal weather and ideal driving conditions.
Or was your point that sometimes you have to drive slower than the limits?
And the lowest common demoninator for driver and machine. Meaning a half blind idiot driving a 1968 land yacht with full drum brakes and worn out suspension on bald bias ply tires will be safe at those limits.
In any case, for instance, many folks mistakenly think that yellow speed limit signs indicate legal maximum speeds. they dont, yellow speed advisory signs are just that, advisory only. Orange ones, however, are mandatory.
Two of my personal favorites are acceleration and decelleration lanes, and divided highway passthroughs. Very few folks actually understand how to use these features.
Accel/deccel lanes- These are for getting up to the traffic speed of the roadway you want to enter. You are not supposed to slowly creep out to the end of that lane and stop, then enter traffic a 20-30mph less than the trafic flow. Or, coming off the roadway, you FULLY enter the decell lane, THEN slow down for your turn. Dont slow to 20-30 less than traffic, then creep over into that lane. In both cases, dont be tentative, make these lane changes smartly, 2-3 seconds at most.
Median Passthroughs - Do not turn into the passthrough immediately, wait until you are on the far side, then turn. This prevents anyone coming from the other side into passthrough the from the opther side of the highway blocking your view on oncoming traffic.