What signifies time zero on the incident timeline?

The correct answer pertains to the moment when the firefighters arrive on the incident scene, known as time zero on the incident timeline. This point signifies the transition from the pre-incident phase to the active incident response phase. It establishes a clear starting point for all subsequent activities, assessments, and operations conducted at the incident.

Understanding time zero is crucial for effective incident management, as it serves as a benchmark for measuring the response time, evaluating resource deployment, and implementing strategies to mitigate the incident. It helps in assessing the operational timeline, from the moment resources are engaged to the completion of incident management operations.

In contrast, other events such as the activation of the firehouse or the answering of a 911 call denote earlier phases in the incident lifecycle. These moments initiate the response process but do not mark the actual arrival on scene where direct actions can begin. Countermeasures starting typically occur after the assessment and upon the arrival, further reinforcing that time zero corresponds specifically to the arrival of firefighters.