Think out of the box, and turn the telescope around.
If you rolled up on a MVA (most likely encounter from your description), what would you like most for a bystander to do before you got there?
First, stay out of trouble. Don't add yourself as another victim or pesky bystander. Park "upstream" on traffic if SAFELY possible and start there, your marker lights and headlights may be of use and you will not block incoming units. Stay away from fire hydrants.
Second, make a fast survey and call or send for help. How many, where are you, what do you see. Also, see #1 above. Have the non-emergency numbers for local FD and law enforcement as well as emergency numbers, sometimes (especically on cell phones) you will be very delayed phoning in to 911. If no cell phone, have phone change, you may have to leave the scene to call for help if out in the sticks.
Third, scene safety. See item one above. Have a safety vest, at least five warning devices which will not start a fire, weather gear (you should dress to change a tire always, or carry it with you). A good hundred yards upstream, begin your warning device pattern in the gutter/on the shoulder and bring it over to the line no farther than 100 feet out, local conditions allowing. If needed, go back to the last intersection and mark it there to prevent cars stuffing a bottlenecked road and blocking arriving units. Look and smell for fuel. Tell people they need to turn off their ignitions and put the keys in their pockets, grab their purses or other essentials and, if they can, get to the shoulder/curb to await help.
FOURTH: begin second survey and then start helping people without unnecessary movment.
Every scene's a little different, and some are a LOT different, but always refer to step #1. As a samaritan first responder your first duty is scene safety and last is trying to engage in medical activity.