Arbitrary Limits

So I have a serious beef with driving, smoking, gun-owning, and drinking limits. I guess I should rephrase that--I don't have a problem with the existence of limits, but the fact that those that exist are so arbitrary. To illustrate: a stretch of road in one state can have the exact same characteristics of a stretch of road in another state but have a different speed limit just because it is a different state. So, just because I cross the state boundary the laws of physics behave differently and what was formerly safe is no longer so? I don't think so! Or just because I go to another state or another country suddenly my maturity level increases to where I can safely drink alcohol on my own at a younger age? Limits are supposed to be for the safety of the people, but its all about what different governments think is safe and not about what actually is safe. Again, why 65 mph? Why not 70 or 80?  The difference between the results of an impact at 65 and an impact at 80 aren't that different--you're still going to total your car and sustain serious injuries. And human reaction time isn't that much worse at 80 vs. 65. Or again, why 21 before I can buy alcohol? Why not 19 or 17? Or younger? Heck, at 18 most young people are done with high school and either starting a job and possibly a family or going to college and higher education--they are not children anymore! What are the criteria for these limits? Again, it seems that no one has really thought this through logically--the limits are set to what the current people in authority think is best, but this may not be what actually is best for their people. And this is just wrong--governments are given their authority by God for the good of the people, and thus they have a serious responsibility to use that authority wisely and for the purpose for which they were given it: the good of the people. Random limits just because there should be limits is not wise use of authority!