Final answer:
The next step after the primary assessment is to immobilize the patient on a long backboard to prevent further injury, followed by a rapid trauma assessment to identify any additional injuries and assess neurological function.
Step-by-step explanation:
After performing the primary assessment on an 18-year-old male driver involved in a vehicle collision and who is complaining of neck pain, the next step is to immobilize the patient on a long backboard. It is critical to assume a cervical spine injury until ruled out with imaging such as an MRI or CT scan. Do not move the injured person as it may result in permanent paralysis or other serious complications.
You should provide basic first aid while ensuring that the person's head and neck are securely immobilized. A rapid trauma assessment is also important for assessing neurological function and identifying any other potential injuries that need to be addressed immediately.