I very much like the countdown idea - a pedestrian would at least know the lights are working. And it would encourage tfl to set sensible waiting times.

In the trade-off between pedestrian and driver inconvenience, my rough rule would be that if 4 cars are going to be held up for 10s, then 1 person crossing should be prepared to wait 40s.
If 8 cars are being held up, then 80s is not an unreasonable wait.
But of course 2 people crossing should then halve the time, and so on.