What instructs air traffic to change airspeed to maintain separation?

The choice is grounded in the primary function and authority of Air Traffic Control (ATC). ATC is tasked with managing the safe and efficient flow of air traffic in controlled airspace. One of their key responsibilities is to maintain safe separation between aircraft, which includes instructing pilots to adjust their airspeed as necessary.

When two aircraft are on a converging path, for example, ATC uses radar and other tracking systems to monitor their positions. If there is potential for conflict, ATC can issue directives for one or both aircraft to change their airspeed, altitude, or flight path to ensure that they maintain a safe distance from each other. This ability to control traffic directly impacts flight safety and operational efficiency, making ATC the authority responsible for issuing these speed adjustments.

Other options do not hold the same authority in this context. Pilots may communicate their intentions and report their situations, but it is ATC that has the jurisdiction to implement changes for traffic management. The Flight Operations Manual provides guidelines and procedures but does not actively manage traffic. Weather Reports, while important for flight operations, are not specific to air traffic separation and do not instruct changes to airspeed for that purpose.