take/send

I think you are correct.

Although it is curious why John needs to go to the hospital since he only saw the accident.

A careless mistake. I've rephrased the question.

John saw a road accident and called an ambulance to send the victim to the hospital.

I think'take' should be used, not 'send'. Am I correct?

Yes, "take" is correct.

If "send" were to be used [note: "were" is subjunctive in this sentence], that would imply that the ambulance would arrive at the scene of the accident, but the victim of the would depart and go to the hospital by some means other than the ambulance.

That scenario is extremely unlikely, but it is conceivable. For example, the ambulance could suddenly become mechanically disabled, and another vehicle (such as a highway patrol car) could transport the victim.

Even there, "take" would be preferred, because that word would be appropriate for an injured and helpless person, whereas "send" would imply that the person still had some independent ability to travel.

Thanks, Aa.