Well, it's one of those estimation questions. The general idea is to come up with a structure for solving it that shows your thinking process and your ability to analyse a problem by identifying its key drivers and stripping it down to its essential parts. A good start to solving this one would be identifying where traffic lights are used - intersections. How many intersections? How many Streets? Just use a small borough/county as an estimate then multiply over the number of counties, get a figure, estimate how many lights per intersection, then you have a figure. Interviewers are more interested in how you arrived at your answer than the final answer in itself which is largely meaningless.