Traffic signals are one of the areas where human drivers add the most delay. I've definitely been in situations where each car takes 3 additional seconds to notice that a light has changed, and cars continue to run the light (e.g. making left turns) long after it has changed. I'm too lazy to run the simulation (https://traffic-simulation.de/intersection.html), but I'm guessing throughput is much better when every driver is obey the rules and paying total perfect attention.