It would also drop global crime rates to zero instantly, since every crime (well, except some of the draconian victimless ones which basically represent a crime to enforce..) explicitly involves discourtesy to people with whom you have a relationship of "stranger" or better.

One way that I like to try to model human behavior is that we are all 2 year olds at heart, but we've built an interesting edifice of coping strategies and discipline and guilt and dysfunction atop that foundation in order to approximate our roles as rational, adult actors. That what we know of as "civilization" is simply the global compromise that has resulted from these efforts.

So, achieving perfect courtesy to every unknown person at all times would either require improving the toddler at the foundation of it all, or perfecting the infrastructure built upon that until it was functionally flawless despite the unpredictable foundation.

The other point of note is that a strategy which requires everybody to faithfully meet a certain guideline is not interchangeable with a strategy to meet a certain guideline yourself despite many other people not meeting it.

The best example would be "trust": you can demonstrate trust in your fellowman by never locking your doors and not hiding your valuable personal possessions, etc. If everybody did this in tandem then all of life would be rendered more convenient.. but so long as anybody strays from that discipline they would be capable of robbing everybody within commute distance and probably getting away with it.