Is that level of complexity actually necessary? The way that the UK system appears to work (as a user) is that the IR sensor detects a vehicle approaching the intersection from the minor road and, if there are no vehicles approaching on the major road, it gives you a green light by the time you actually reach the junction.
I agree that the app-based system would theoretically be slightly better in that it has more information to work with, but given that we're basically talking about a stochastic process then it feels like the IR system should really be good enough.
It's not strictly necessary but it takes giving a crap and effort and constant auditing and reassessing to do all that light timing well with the current state of technology. Outsourcing that responsibility to centralized computer that can do most of it automatically and on a schedule is likely a huge improvement for the quality of programming/timing for the average light if only a neutral for the best ones.
I agree that the app-based system would theoretically be slightly better in that it has more information to work with, but given that we're basically talking about a stochastic process then it feels like the IR system should really be good enough.