IN A CAR accident, your best ally is your smartphone: Document the scene. Take photos or video of your car and other vehicles or objects affected, says Sean Tucker of Kelley Blue Book. You can also record a voice memo to recount the events and use the Notes app to type in contact info for other drivers or witnesses. Connect with your insurance company; most have apps to report an accident, says Bob Passmore of the American Property Casualty Insurance Association. Be sure to download yours before you need it. —Brett Berk