What is the biggest hazard when passing parked cars?

The most significant hazard when passing parked cars is indeed the risk of a car door opening. When you are riding a motorcycle or driving a vehicle near parked cars, you must be particularly vigilant about the possibility of a door unexpectedly swinging open into your path. This scenario, often referred to as "dooring," poses a severe risk, especially for motorcyclists who have less protection compared to larger vehicles. The rapid nature of passing parked cars combined with the potential for sudden movement from those in their vehicles can lead to serious accidents if a door is not seen in time. Always maintaining a safe distance from parked cars helps to mitigate this hazard effectively.

The other options present risks, but they do not encapsulate the immediacy and danger associated with an unexpected car door opening in the same way. For example, a car pulling out is certainly a concern but usually provides a clearer visibility of its motion compared to a door that can open suddenly with little warning. Similarly, a person stepping from between parked cars and pedestrians walking on the sidewalk pose dangers that can be anticipated with careful observation, whereas an opening door can occur without notice, resulting in a much more immediate risk.