"Watch out, I'm on your left lane!" or 'get the fuck out of my lane, asshole!', or something like that!
Actually, that's similar to something I was thinking about one day.
Like, in some not-too-distant future (Next Sunday, AD?), self-driving cars actually talk to each other. We get in, we program our destinations, they calculate the route to take, and off we go. But when on the roads, commute distances differ. Some people might get on the interstate and get off at the next exit. Others might be on the interstate for 20 miles. But the cars talk to each other.
"I'm getting on the interstate, but I'm going to be here for a while."
"Okay, we'll make room for you so you can get into the cruising lane."
And then 2+ vehicles adjust their velocities to make room for the car that's got a long way to go.
"Hey, I can see there's a lot of us over in the slow lanes, but my human's exit is coming up in 0.25 miles."
"Okay, here's a spot you can use so you can make the exit."
Maybe this would just be some self-driving car equivalent of an ad-hoc wireless network. I don't know. Just seemed like a way to ease road congestion without requiring human intervention.