Go out earlier is a practice that I think it is not done anymore. If I have a meeting at 4pm and I know that it takes me 30 minutes to get there, I go out of my house one hour before. This is for answer your doubt about how would you let know to your friend that you will be late, you weren’t able, the best you can do is be preventative.

Imagine you send a letter to a fellow who lives overseas, you expect to see him september the 3rd in a concrete place. I can bet that person would arrive like a month before and wait the right time. (Obviuosly would not wait for you in that very place, that person can stay in a hotel or whatever).