I live in a part of the country where the traffic is heavy and un-predictable. So I try not to make too big of a deal about people showing up late, as long as they call and let me know the situation. A few of my relatives are notoriously late to every event they’ve ever attended. We know this about them and plan accordingly.
It completely depends upon who you are visiting and why if you are going to show up early. If you are going to a big dinner party, it would be rude to show up early because the hosts will likely still be getting ready and they shouldn’t have to entertain you during that time. I too, have sat in my car and read the paper or listened to the radio for awhile if I’m early. And because of the traffic out here in Southern California, it happens often (both early and late).
If it was my best friend and we were just going to hang out or go to the movies or go out to lunch, she wouldn’t care if I came early. I would just play with her cats and converse with her from the other room if she was still getting ready. No biggie.
You should show up to work, at least a few minutes early, so there are no problems. If you got stuck behind a huge accident on the freeway, you should call your boss immediately and then expect to work overtime to make up the difference.
If you are meeting someone for a date (especially for the first time) you should show up at least a few minutes early. That way you have time to park, to use the restroom, to walk from the parking lot, to look for the exact address or whatever.
If it was a big casual family party or big casual picnic with friends, showing up late or at any time while the party is still going is fine, at least it is with my family and friends. One of my cousins is an emergency room nurse, so she has weird schedules of being on and off work for extended days and her husband is a firefighter with a similar odd schedule. We usually have our casual family parties on the weekends so it’s convenient for most guests, but we expect that these 2 may not be able to come until the party has been going strong for a few hours, or may be able to stop in at the beginning to say hello and then have to leave, but we’re always grateful for their company.
I think if I had a friend (especially and S/O) who was regularly late without a good explanation, especially if they made me miss events like concerts, movies, dinner reservations or plays that have a specific start time and for which I’ve paid money, I would have a serious discussion with them and maybe stop planning on going to events with them.