What is the role of queue detectors in drawbridge preemption?

The role of queue detectors in drawbridge preemption is to measure traffic congestion ahead of the bridge. This is crucial because queue detectors collect data on the number of vehicles waiting at a traffic signal or approaching the drawbridge. This information allows the traffic management system to recognize when congestion is building up, enabling timely preemption of the traffic signals to facilitate the drawbridge opening.

By monitoring traffic conditions in real-time, queue detectors help ensure that there is a smooth flow of traffic, minimizing delays for vehicles and preventing excessive buildup before the bridge opens. This efficient management is especially important during peak times when traffic is likely to be heavier, allowing for better coordination and scheduling of bridge openings to reduce interruptions.

In contrast, the other options focus on roles that do not align with the primary function of queue detectors in this context. For example, detecting vehicles after the bridge opens or controlling the opening speed of drawbridges do not pertain to the key information that queue detectors provide, which centers around vehicle counts and traffic conditions prior to the bridge.