What do you do if you are in it, and what do you do if someone else is in it but you witnessed it?
i am an EMT, what you do is stay with the vehicle if possible, if you are outside the vehicle it is possible that there may be secondary crashes form other motorists not paying attention. stay buckeled up and call 911 immediately. if you do get out- move away from th vehicle on the ditch or median side and stay away from it. assisting others in getting out of a smashed vehicle can possibly make their injuries worse, our rule of thumb is that if the airbag in a vehicle has deployed then the accident contained enough force to cause significant injures in those involved. best thing to do is call and wait for the pros to show up, it can be very dangerous getting people out of vehicles, especially if the airbag has not deployed- it can suddenly go off while you are in the way and cause injury to you and the other person- airbags come out with alot of force. so stay out of harms way and wait for the proper emergency people to show up. use your own judgement- there is a 'good samaritan' law in most states that says if you help someone in good faith and they are injured further- you are not liable for that injury and can not be sued. if you believe that someone will die if you do not act before emergency people get there- then do what you feel is appropriate.
I was in a car, bad car accident with my 7 month old daughter in my arms. First I learnt to do in that situation is that you muct remain calm. Try not to make a sudden move which may worsen your wound or anything. Try to look around in a clear mind and try to get the fastest way out of the car. This is important just in case that the car might blow up. As soon as you get our of the car, reach your mobile and make a call to the police is the first thing you must do before trying to give help to other victims if there are any other people in that car. If you wintness a car accident, calling 911 must be done first. Try to come closer and make the people in the car to stay calm before you CAREFULLY try to give help by taking the victims out, if POSSIBLE and if YOU ARE SURE that that is possible. In a worst condition you should wait for the pros to help them. Calming them will give great help.
Many years ago my father-in-law was in a multi-car accident where one of the parties got estimates on fixing a different car from the one that was in the accident. This was before cell phone cameras but when several witnesses gave a description that didn't match the vehicle he was caught. The guy still tried to sue everyone involved in the accident. The court assigned 20% of the blame to the driver suing, 80% to a semi driver, and 0% to my FIL. After that I always made a point of carrying a camera in my glove compartment but now of course cell phones have cameras which will do. Funny thing is that I wasn't in any accident until I was 35 and I blanked on everything. Luckily the guy who rear-ended me was very nice and more embarrassed than anything else.
In all cases, get license numbers, names, and insurance companies. Call the police, even if there are no injuries. Call 911 if there are injuries. If you are just a witness, if can stay around, talk to the police, if not, give someone your phone number so they can contact you. And most importantly, DO NOT discuss the accident with the other people. Wait for the police and answer their questions honestly. People will be upset after an accident, do not get into any argument or discussion, remain calm and wait for others to arrive and sort things out.
One tip I've heard (thank goodness, I've never had to use it), if you're able too - take some pictures of the cars with your phone (assuming it has a camera on it). These can be valuable when it comes to insurance etc. Of course, if you're hurt or the other people are hurt, this wouldn't apply (more applicable to fender-benders).