First time I ever stopped at an accident, I'd been a paramedic for a couple years already. I only stopped because it was a pretty bad looking wreck and there was no one there yet. Just wanted to see if I could help somehow. I didn't realize it until I walked up, but the scene was already swarmed by off-duty Rickey Rescues who had "taken control of the scene" and wouldn't let anyone else near that patients. Last time I ever stopped.
Until about 10 years later, when I was driving down I-77 in Virginia and saw a pickup from the oncoming lanes careen across the median, go airborne, roll a couple times, and come to rest on it's roof. It was weird because for years I'd responded to hundreds of scenes that looked just like it, but had never actually witnessed anything more serious than a fender bender. I pulled well off the highway maybe 50 meters away, and in spite of getting to it no more than probably 90 seconds after it happened, it was, again, already swarmed by Rickey Rescues. It's like they just appear out of thin air. Anyway, this time they were just standing outside the truck pointing through the passenger side door at the driver, who appeared unresponsive and was hanging upside down from his seatbelt with his head against the roof of the truck, and his neck flexed hard with his chin crammed into his chest. He didn't look like he was breathing. I asked if anyone had a knife on them, and one of the whackers did. I crawled through the open passenger side door, and the guy wasn't breathing. I did my best to position him in a way it looked like he would fall into a position where I could pull him out rather than in a contorted mess that would trap us both, and I cut the seat belt. It worked, thankfully. He fell the right way and I pulled him out and opened his airway, and he was now breathing just fine. He was completely unresponsive and was a big fat guy and I could only maintain his airway by putting him on his side, a la "recovery position". It seemed like a long time until the engine arrived (was probably 5 min), and in that time I was interrogated by a dialysis nurse about my qualifications to be "in charge of" this patient, and why didn't I have him on his back, holding his head still? Fire arrived and seemed very apprehensive about taking a patient who was unresponsive to pain and had airway compromise, just had the deer in the headlights, I-don't-know-what-to-do look that new EMT's all have. The firefighter didn't seem as though he wanted to take over, so I just kept maintaining, figuring the ambulance would be there momentarily. Until a cop told me to get out of the way so the firefighters could do their job. I made the firefighter take the guy's head and showed him how I was maintaining the airway, and walked away. As I drove past a minute later, they already had him supine on a backboard.
The experts seem to come out of the woodwork for MVC's, so I would not stop again unless I was in a real rural area where there was no other help around and it looked like EMS might take a while.