Proof Of Fault
If you are in a significant accident you need to secure good evidence at the scene of
the accident to prove your claim. That means not only exchanging insurance information but taking photos if you can of the damage to both vehicles involved and the accident scene which may show who is at fault. You should also get witness contact information and ask what they saw. Of course you must call the police if you are injured or if the accident is a significant one where your car is totaled or badly damaged. The police will investigate by looking at the debris, talking to all parties and witnesses and form an opinion on who is at fault and why. If you can get the other driver to admit they caused the accident maybe record it with your cell phone.
All of this is crucial for your claim because later on the other driver may change their story and blame you.