As a bus driver, it's your responsibility to remain calm and collected behind the wheel. The hiring authority needs to know that you can handle traffic congestion with ease. Mention the cities you have driven in, and what type of traffic you have handled in the past.
If you are a newer driver, it is okay to be a little uneasy about driving in bigger cities with a lot of traffic. The interviewer will likely provide you with a few extra ride alongs to ensure you are comfortable before being sent out alone. Rather than stating your are uneasy or stressed out in these situations, let them know you are less experienced in this area and would appreciate additional training.