Basically if you see a car accident, and there is either an ambulance there or there are already people stopped, you are supposed to drive along and not impede anyone or join a leering crowd. If they need your help, they will wave you down.

If you see for example an upturned or crashed car that no one is around, you are supposed to:

• Stop safely. If you can’t, stop where you can and call emergency services.
• Use the emergency triangle in your car to indicate a possible road hazard at least 50 meters in advance.
• Determine if there are any personal injuries, even suspected (if someone fell from a motorbike or was hit by a car, maybe got a concussion, don’t let them continue driving), with proper triage. First concern is life-threatening injuries, then lighter injuries, disregard people who are obviously dead or not going to survive. “Not going to survive” means missing major parts of body, “not breathing” is obviously life-threatening, but that’s why you know CPR.
• While doing the first three, call emergency services as soon as it’s feasible, start the call with your position, then describe the event, in particular the number and kind of injuries, then tell them your name for reference.
• Start first aid as you can, and follow the instructions of the emergency services.

At least that’s what the driving license exam has you train for.