When you pull over, call 911 and tell them your exact location. They will want to know the nearest cross street, so try to determine your location as soon as possible.
Stay on the Scene and Offer Your Statement to the Police
Once the police arrive, one of the officers will likely approach you to make a witness statement. Your witness statement should include details that help the police determine the at-fault driver.
Seeing an accident occur can be scary and may even be traumatic for some, especially if it is severe. Consider speaking with a therapist or doctor if you find yourself struggling to deal with what you witnessed.