Spend a little time researching and you'll find many accounts of Good Samaritans being killed while trying to help MVA victims. Think about it - what is, by far, one of the most common LODD causes for EMTs, firefighters, and cops besides being in an vehicle? Getting hit by one.
Do your family a favor and take that risk only when you have the advantage of a real emergency vehicle/lighting, other responders, and worker's comp / life insurance/etc.
This ^
There's a reason my department dispatches suppression apparatus along with an ambulance for every MVA, be it on a highway or even at a regular intersection. They block the scene from traffic, and also angle the apparatus so that it will roll away from the scene if struck. We've had several incidents in my dept where drunk drivers have hit engines that were blocking the scene, at speeds high enough to total their vehicle in one case. Imagine if that car ran you over instead.
When I worked in NYC, we only got a fire response if it was a pin job, or if there was a need for a wash down. We were out there alone, and I've had my share of close calls. I've jumped out of the way of a car a few times, and even got whacked in my keester by a mirror as well.
Think about it, what are you really going to do at an MVA? Hold their head? If they're bleeding profusely, they're probably not going to make it anyway, if that district is so rural that a 911 response is many minutes away. I hope you don't have a spouse and kids in the car, and you leave them on the shoulder of a busy highway (in the car) while you go be a hero.
This is what happens: You get out of your car, hopefully don't get hit, get into the car and hold their head. The ambulance gets there, says thank you (maybe), dismisses anything you might try to tell them, such as the pt's complaints, hx, pulse rate, whatever. Then one of their crew takes the head, and you're out of the picture. Hardly glamorous if you ask me. I know that I'm not going to use anything some whacker tries to tell me at an MVA. I'm going to start my assessment from the top.