Unfortunately, it also encourages drivers to gun it when there's only a few seconds left on the countdown ("I still have seconds to get through the intersection!") - which increases the number of cars running red lights.
The countdown is only shown when the pedestrian crossing is on green. Drivers don't get a countdown on their green signal. Most also have a plastic shield around the countdown so you can't see it from where the driver is sitting to avoid people preempting the signal change.
This works on crosswalk-style crossings, but not on 4-way intersections.
Incorrect, It is used in London on thousands of junctions, hundred of 4-way intersections.