It's one of my favorite things about living in a suburban area. Someone will drive down the block, or be walking by, and I legit have no idea who the hell they are as I'm not super neighborly - but I'll still do the little head nod thing and the half wave, and they see it and do it back, and it's just a nice little way of saying "hi, yes, we're in each other's extended personal spaces but have no plans on murdering one another - have a pleasant day."