What specifically helps to expedite traffic flow at busy airports?

The correct answer is based on the function of Standard Instrument Departures (SIDs). SIDs are specific routes designed for aircraft to follow when they depart from an airport. These procedures provide a structured path that helps streamline the movement of aircraft into the busy airspace surrounding high-traffic airports.

By utilizing SIDs, ATC can efficiently manage departures by predefining routes that reduce potential conflicts between aircraft as they take off and begin their climb into congested airspace. This not only minimizes delays but also enhances safety by providing clear, prescribed pathways for aircraft to follow, which can help prevent inadvertent deviations and enhance situational awareness for both pilots and ATC.

While other options may have their own roles in air traffic management, SIDs are distinctly focused on expediting departures, which is crucial for maintaining an orderly flow of air traffic at busy airports.