I think you're probably trying for one of those moral relativism fallacies (why don't we just go around killing people?) but you actually picked a ridiculously easy target that supports my point. How many instances are there of speed limits being set to create speed traps rather than to actually promote safety? Or set based on oil embargoes in the 70s and just left there? Or rules based on nimbyism and not wanting traffic in certain neighborhoods? Or lights timed to catch people at red light cams. This is all off the top of my head, I'm sure I could go on, but we should definitely be skeptical that many traffic rules are made "for our safety" and not for some other agenda, good point!
I don't know. How many are there?
Are you suggesting that the fact that neither the towns nor Kansas has altered the lane width in these locations proves that these speed limit changes are nothing but speed traps?
It is a rule of thumb. Not a universal law.
I hope you are not questioning the motives for having traffic rules. Or motives of your wife asking you to avoid a particularly dangerous place when coming back from work.