How are one-way preferred routes depicted on an airway?

One-way preferred routes are depicted on an airway using a bold arrow. This representation is crucial as it helps pilots quickly identify the specific direction and path they are to follow within a controlled airspace, particularly during instrument flight. The bold arrow ensures that the information stands out clearly against other navigational elements, minimizing the chance of misinterpretation while in flight. Bold arrows are effective for indicating movement direction because they convey immediate understanding of the intended course, which is vital for maintaining orderly traffic flow and safety in busy air routes.