Around here there is some kind of law about stopping when you are off duty; if you are in a car with some kind of EMT identification (i.e. sticker thing on windshield), you are required to stop and offer assistance if police or other rescue crews are not already on scene. The other day I was with a fellow EMT, and we heard on his scanner about an MVA down the road, we decided not to stop since the ambulance should be ariving in a minute or so. As we kept driving, we saw the MVA; it was in a different location than where the ambulance was dispatched to. So we stopped, told the cops to radio dispatch, and offered help. We took initial assessment, baseline vitals and c-spine by the time the ambulance got there. We told them what info we had, and stuck around (out of the way) to see if they needed any extra help. Turns out both patients need to be back boarded, and we were around to help load the patients. If I stop on scene, that's what I usually do: what I can until the ambulance gets there, and then offer help any way I can-if they don't need it, I leave.