I was watching this video of a live chicken trapped on a moving truck and thought it was strange that it’s not possible to say anything to them even when circumstances might warrant it. All we got is honking and waving. There could be a touchscreen interface with a map of nearby vehicles. It could be voice controllable or the passenger could do it for safety.
Maybe a manual dial to cycle through the available nearby vehicles then. The idea is just that there should be a way for it to be clear who you are contacting and where their vehicle is on the road relative to yours.