What is the process called when normal signal control is switched to a special mode?

The correct process described in the question is known as preemption timing. This refers to the method by which normal traffic signal controls are altered to prioritize certain types of vehicles, such as emergency vehicles, trains, or other priority functions.

Preemption timing ensures that these priority needs are met effectively, allowing for a specific mode where the typical signal operation is suspended in favor of enabling the passage of vehicles that have a critical need to proceed through intersections without delay. This is particularly important in scenarios such as fire trucks responding to emergencies or public transit vehicles on scheduled routes.

The other terms provided, while they may seem related, do not specifically define the act of switching normal signal control to a special mode focused on priority vehicles. Signal override typically suggests a manual action taken to bypass normal sequences, traffic control encompasses the broader management of all traffic, and signal transfer refers more to the movement between different control methodologies rather than specifically addressing priority signal operation. Understanding preemption timing is crucial for traffic signal technicians as it plays a significant role in managing public safety and traffic flow in urban environments.