Every driver has a cellphone in their pocket, I'm sure it's more than possible to integrate people with modern vehicles in a mesh net to older vehicles by using an app or something. It's not hard to think about, c'mon now. What website am I on ffs?
The problem is reliability. The cellphone is not constantly connected to a network, depending on its location, battery, age of electronics, software updates,... You would need a system that works with a mesh with holes. Not much better than today's situation i would guess.