Which of the following is essential when checking for traffic before exiting an emergency vehicle?

Looking in mirrors for oncoming vehicles is essential before exiting an emergency vehicle because it helps ensure the responder's safety. Before leaving the vehicle, checking the mirrors allows the responder to become aware of any approaching traffic, including fast-moving vehicles that may not have adequate time to react. This awareness helps prevent accidents, as exiting a vehicle into oncoming traffic can put the responder and others at significant risk.

The other options do not provide the same immediate safety benefit. While checking the vehicle's speed is certainly important for understanding overall traffic conditions, it does not directly assist in the immediate action of safely exiting the vehicle. Seeking assistance from other responders can be useful in various scenarios, but it does not replace the necessity of ensuring that the immediate area is clear of traffic. Using a traffic control sign can help manage oncoming vehicles but does not replace the need for the responder to personally verify traffic conditions before exiting. Thus, looking in mirrors specifically addresses the immediate concern of safe exit from the emergency vehicle.