3. Good drivers will ignore the limits if they are unrealistic.
Nonsense. Good and bad drivers do not know why a certain limit is where it is. Good drivers keep to the limit for exactly this reason and can try to change a limit if they feel it is unrealistic. Bad drivers don't give a shit one way or another and should have their cars impounded, their licenses revoked and issued a bus pass.
Oh really? They look like arbitrary numbers to me. Like here, they never place a sign, which starts with an even number. Always 10-30-50-70. God knows why. Probably because that how they are marked on the speedometer (IDK, I have a digital number on it), and the 1 bit user has hard time setting it between two lines. On my commute way, in the last 2 years, 5 sign was changed. I mean reduced. Yesterday, the road was fine till 70, today it isn't. Nothing else changed.
The best reasons to reduce the speed limit is usually "but there are too many cars here".
Yes, because give me an alternative. No? OK, then I take the car. And because you reduced the speed limit, I will spend MORE time there. Now there will be even MORE cars, because of that. And some of them will be speeding. Good reason to reduce the speed limit.