Which of the following is NOT a common mode of operation for timing plans in a signal system?

The timing plans in a signal system are typically designed to respond and adapt to various traffic conditions to maximize efficiency and safety. The modes of operation include time of day, which operates on preset schedules based on historical traffic patterns, traffic-related plans that adjust based on real-time traffic volumes or conditions, and traffic-responsive systems that dynamically change signal timing in response to detected traffic demand.

Emergency operation is not a standard timing plan mode. While signals do have protocols to respond to emergency vehicles—such as those equipped with transponders that can temporarily alter signal phases—the term "emergency operation" itself does not refer to a routine mode of timing. Instead, it is an exceptional consideration rather than a consistent operational mode like the others listed. Therefore, recognizing this distinction clarifies why emergency operation is not categorized in the same way as the other common modes of timing plans for a signal system.