What to do if you witness an accident
There are some stories about foreigners stopping at accident scenes, trying to help and ending up getting the blame. So some people, because it happened to themselves or to some friends or because they read the stories on the net, have decided that they wouldn’t get involved and wouldn’t stop at an accident scene. I say it’s up to them and their conscience, but I know I’d rather live with a grudge than with guilt. So my advice is, stop if you can make a difference, that is stop if the accident has just happened, or stop if first aid is needed and you can give it. If the accident seems to have happened some time ago and the situation is being taken care of, no need for you to hang around.
Nothing different than anywhere else: secure your own vehicle, call an ambulance if you can speak Thai or have someone do it (see emergency numbers in Thailand), secure the scene as much as you can, and give assistance and first aid if you have been trained to.
There is no reason why you should be blamed for an accident you weren’t even in, but let’s face it, it’s not like it never happened. If you want to avoid any problems, buy a dashcam. It’s a video camera that you put inside the car and that records everything that happens in front of you. The other thing you should do is work on your Thai, as speaking Thai enables you to understand what people say (obviously), and to defuse the situation before it goes out of control. People who have no clue what is being said around them are in a vulnerable position.
What to do if an accident happens to you?
The first thing to do, if hopefully you’re not seriously injured, is to call your insurance. All insurance companies have a national call number, and they will send someone at the accident scene to take pictures and negociate. In the meantime, it is usually recommended not to do anything. Don’t speak, don’t admit anything, don’t move any vehicles (unless if the police is asking you). You may take pictures, but basically you want to wait for the insurance guy.
Whether you caused the accident or not, you may have to take the blame. If a drunk guy crashes his motorbike at high speed into your car and he dies because he didn’t wear a helmet, you may have to take the blame. You were there, and you can pay, that’s the two decisive factors.