To see a relative or friend getting into an accident means that you may have a conflict with that person, but you are still keeping things under control.
To see an accident is not a good sign. To get into the car accident means that you should be ready to unpredicted situations. If you avoid the accident at the last moment means that you will be able to avoid a collision with your enemies.
To see an accident means that you might be pulled in the legal claim or your spontaneous actions will destroy your plans. To be a witness of the accident, but not to be involved in it, means that you will get unexpected profit due to mistakes of other people. To help someone, involved into the accident means that you will get news from the friend in troubles. Railway accident means money loss. To see crashed cars, or someone injured in the accident, means you should not rely on other people, if you want your plans to be realized in a way you want.