Which action is least effective at increasing a motorcyclist's visibility?

Wearing light-colored clothing, riding with headlights on, and making eye contact with drivers are all actions that significantly enhance a motorcyclist's visibility to other road users.

Wearing light-colored clothing increases visibility, especially in low light conditions or when contrasted against darker backgrounds. Riding with headlights on, even during the day, helps to draw attention to the motorcycle, making it more noticeable to drivers. Making eye contact with drivers is an effective way to ensure that the motorcyclist is seen and acknowledged by other road users, which adds a layer of safety to interactions on the road.

Using loud exhaust, while it may alert other drivers to a motorcyclist's presence through sound, does not improve visibility in the visual sense. Sound alone cannot substitute for being seen, as visual cues are critical in traffic for safe navigation and awareness. Thus, this option is the least effective in terms of increasing visibility to other drivers.