Final answer:
The first action to take upon arriving at a major car crash is to get appropriate backup and emergency personnel en route to the scene to ensure proper management and safety for all.
Step-by-step explanation:
When you arrive at a major car crash on a highway, the first step you should take is to Get appropriate backup and emergency personnel en route to assist you. This is because establishing a quick response team and ensuring support is in place is crucial for effective management of the accident scene. Once backup is secured, other steps such as creating a safe work environment, assisting those in immediate life-threatening situations, and treating and transporting victims according to their emergency priority can follow.