The way I've seen it work is that there's a touch screen in the elevator lobby. You tap a floor, and then screen will say "Car C" (along with an audible message). The problem is that then the system can't reassign you to a different car after that. You'll typically have a bunch of different people going to a bunch of different floors, and there would be no reliable way to communicate to each individual their new assignment. Each car will also typically display which floors it's going to when it arrives. I suppose that, rather than assigning a car immediately, you could have passengers check cars as they arrive to see if they're going to their desired floor. Then the system could do reassignment, but people would be scrambling to check every car to see if it's the right one for them, and that's assuming that they understood the system. It would be way too chaotic in practice.