It depends on how catastrophic the accident is.
If you’re on a nearby roadway, try to safely get yourself off the roadway to a safe place. Get yourself out of harm’s way, but obviously don’t leave the scene.
Should you call the police?
Call 9-1-1 immediately if you’re seriously injured. (If someone caused property damage only, however, I wouldn’t call an emergency phone number).
Should people pull out their phones and start recording the scene?
When somebody is seriously hurt, I don’t think that should be their priority. Health and safety come first. Under most circumstances, emergency responders will take photographs, take measurements, identify and interview witnesses, and provide emergency treatment at the scene. Only after you are safe and your health is not at risk, take photos and preserve evidence.