A doctor no matter how good does not have x ray eyes maybe a portable ultrasound only as now carried by HEMS London. Initial management is directed to securing an airway if required by endotracheal intubation or in certain cases a surgical airway and then ensuring adequate respiratory function, if necessary using bi-lateral thoracostomies for thoracic trauma. Spinal stabilisation and fluid replacement being either I/V or I/O routes. This all takes time and is eating up the 'golden hour' of improved potential survival.
An instance, unknown person who is unconscious and fitting, what is the cause, trauma drug overdose brain haemorrhage or emboli or epilepsy, all are possible and the doctor would have to treat what they see for the patients safety, definitive diagnosis is up to the trauma team following handover and further investigation. The above could require surgery, a trip to a cath lab, thrombolytic therapy, detoxification or sedation and further drug therapy. Difficult to diagnose at the roadside.