What's interesting is that cities have found that countdown timers cause drivers far up the block to race to beat the green light before to turns yellow/red, which then causes collisions with bikers and pedestrians.
I haven't seen this behavior with cars; but, as a cyclist, I use them to figure out if I can make it through an upcoming intersection or not.
What's annoying is the lack of a standard action when the countdown timer reaches zero. At some intersections, the timers are set so that the yellow light is the last second or two and zero is when the traffic light changes to red. Other hit zero and then the traffic light changes to yellow. Some hit zero and a number of seconds pass before the traffic light changes to yellow.