If you see dream imagery of yourself in a car crash, it can mean several issues. The most obvious would be a reference to your driving. Are you a careful driver? If not, this dream is a reminder to be cautious while driving.
If you see dream imagery of someone else in a car crash, you should enquire after them, in your waking reality. Dream imagery of a car crash is a warning of the bad situation. If you are able to help, you might avert the car crash. You might have seen some stimulation a of the bad situation during your waking reality. This explains why you have dreamed about a crash.