You let someone in, allow them to get in front of you in line to wherever you are going. It only means a car length to you, but it could mean several to them as they wait in a driveway looking at a packed roadway. Sometimes it is on the freeway when someone didn't plan ahead enough, or maybe they intentionally sped past the backup to get to the front of the line. Hard to tell their intentions behind glass and steel, but if there is no courtesy wave, it is more likely the latter.
You come to expect it as a small acknowledgement of the courtesy you extended them. Sort of a social contract, a 'thank you' for an unspoken 'please'. When you don't get it, you don't necessarily regret letting them in, but it does feel like they dropped the ball on their end.
I received one of the most effusive courtesy waves this afternoon. A car swept out in front of me today, left turning into my lane without so much as a glance in my direction. I didn't have to skid to a stop, but evasive maneuvers were necessary. I gave her a one second blast of my horn to let her know I was there.
Almost immediately her hand went up to wave an apology. "Sorry, didn't see you, my bad, I swear I am not normally a jackass." We pulled up to a stop light a couple blocks later, and after a moment, up her hand went, waving another apology. I waved back, "no big deal, just be more careful."
The light remained red, and after another ten seconds, she stepped out of her car. She turned to me, patted her heart, and with regret and embarrassment on her face mouthed another "I'm sorry." I smiled and waved to her again, no damage done. And thanks.
After I parked my car and walked to the bookstore, still smiling from the strange exchange, I thought back on a conversation with a friend a few years ago. We were driving together, and he made some sort of inattentive move and cut someone off. After waving our own apology, we drove on.
We got to talking about it, and agreed there should be some sort of parole period after making a boneheaded move. A moratorium on getting angry at other drivers for six months, realizing that everyone makes a stupid mistake now and then, having just proved it yourself.
The hand on the heart apology will pop into mind the next time someone cuts me off, and will earn the next person a little more understanding in our little social contract.