Had a guy on my old block who did the same thing and everyone thought he was just nosy. Then we found out his wife had passed a couple years before and waving at cars was his way of still saying hi to people every day. Dude knew every single car on that street, like he could tell you who was late for work by 7:15. Took me way longer than 8 months to figure that out, probably like two years honestly. Now I wave at everybody like an idiot. Never know what someone's going through.