Having grown up in the New York City area, I would be wary of this being a set-up. My instinct would get me wondering whether there were accomplices hiding behind the rocks or bushes, ready to jump me if I get out of my car to help.
Also, I have no expertise in aiding a severely injured person. Stopping would just be a feel-good experience for me, but it might be the worst thing I could do for the victim (if the person really were what they seemed to be.)
I would drive on to where I could make a phone call. Perhaps I would hit the brakes, roll down the window half way, and call out that I was going to call for help, before driving on.