Before anyone tries to do anything quite as sweeping as this, here's something much simpler which might well lay a solid foundation for cars talking to each other. Why not start out by having them talk to traffic signals? If at an intersection where, say, the North-South road has the green light and there are no more cars waiting to go, why not give the 15 cars waiting on East-West that are stuck on the red the OK to move? In other words, dynamic adjustment of the light cycle based on the situation. Not only are the technical problems a whole degree of freedom simpler, but the legal challenges of writing the laws to cover the changes will be simpler, too.