What signals the Incident Commander to call for an evacuation?

The correct choice for signaling an evacuation is the use of three long blasts of an air horn. This signal is widely recognized in emergency management and firefighting protocols as a clear and distinct way to communicate the need for immediate evacuation to all personnel on scene. The use of long blasts is effective in loud environments, ensuring that the message reaches firefighters and any civilians in the area who may need to evacuate promptly.

Using three long blasts has become standardized in many incident command systems, as it differentiates the evacuation signal from other sounds or warnings. Emergency responders are trained to recognize this specific sound, allowing for quick action in potentially dangerous situations, thus enhancing safety and minimizing confusion.

In contrast, the other options, while they may involve audible signals, do not carry the same level of recognition or standardization within emergency operations. A short horn blast or two loud sirens might not effectively convey the urgency or nature of the situation, and a verbal warning over the radio could be subject to misunderstanding or not reach everyone due to communication failures or noise. The clarity and reliability of the three long air horn blasts make it the preferred choice for evacuation signaling.