Collision in the dream has different meanings according to what you have seen. To see a collision between two cars in your dream indicates that you will witness a sudden change in your environment. It may be a shocking situation. If the cars are both badly damaged, then the dream represents the collision will bring about a completely new situation and everyone has to get accustomed to it.
To dream that you are in the car while it is crashing something indicates that your lifestyle, experiences, goals or beliefs are not suitable for the society that you live in. It will be painful to keep up with variety and novelty. If the collision is between you and a car, then the dream may denote that there is a conflict in your life and you can not cope with that conflict. You have to change yourself.
Alternatively, a collision in the dream may symbolize that you will face some difficulties in a short time. The dream can be considered as a warning for you. You will be in some dangerous situations and you should take measures to protect you from unexpected things. It may be related to your driving habits. If you drive carelessly, this may cause an accident.
In addition, collision does not have to be between vehicles. It may be between you and someone else. To see that there is a collision of opinions in your dream may symbolize what you have been experiencing in your family life or at your workplace. The dream suggests that there is a disagreement which you want to solve and this situation affects you.