Most of the functions of my car GPS are inoperable when the car is in motion above a certain speed threshold. I'd like to see the same for cell phones, whether bluetooth connected or not, i.e., they can't receive or transmit if significantly in motion.
The brain can really only think about one thing at a time. If you think you are multi-tasking, in reality what you're doing is switching between thoughts rapidly. The key is whether or not you are really in control of the switching process. I think that control decreases when conversing with someone who isn't present. Even then sometimes I tell my wife in the passenger seat that I can't talk and need to focus. For some reason women have a bigger problem accepting this. I think their brains switch more rapidly.