>and given global knowledge of where people are and where they want to go, it could plan the absolutely most efficient route.

Random digression, but I think you will find this is an NP-hard problem - I'm quite sure you can encode the Travelling salesman problem[1] within it. So the absolutely most efficient route is probably not feasible, although the system could certainly do a good job optimizing over what we have now.

[1] http://en.wikipedia.org/wiki/Travelling_salesman_problem