Which of the following is not a cause of increased stopping distance on slippery roads?

Driving with the lights on is not a factor that increases stopping distance on slippery roads. The primary purpose of headlights is to enhance visibility for the driver and to make the vehicle more noticeable to other road users, particularly in low-light conditions or adverse weather. While having your lights on is crucial for safety, it doesn't physically impact the traction or braking capability of the vehicle on slippery surfaces.

In contrast, wet conditions, worn tires, and excessive speed all directly relate to the vehicle's ability to stop effectively. Wet conditions reduce friction between the tires and the road surface, leading to longer stopping distances. Worn tires can also decrease traction, making it harder for a vehicle to grip the surface, which again increases stopping distances. Furthermore, excessive speed diminishes the driver's ability to react timely and increases the distance required to stop safely. All these factors contribute to a significant increase in stopping distance in slippery conditions, unlike driving with the lights on.