I have a car camera installed in the windshield. You can get them for $100. There are situations where, unless you have a witness, responsibility is shared between both drivers and you are both at fault: you're driving along and somebody sideswipes you into your lane. Without a car cam, it's impossible to prove it wasn't your fault.
There are also scams out there, such as a person who stops on the highway; you stop behind and they back in to you, then they accuse you of rear ending them. Then you get sued for whiplash etc. When you point at the car cam, they scamper. Then you call the police with your video.
Car Cams are very popular in Russia, because cops tend to be corrupt. Every car made and sold in China is equipped with a built-in car cam.
I got rear ended a couple of years ago, while making a turn into a driveway. Although the other driver was cooperative, looking back at the video file, it was evident as it showed the car lurching forward. Then I took it out of the car and video'd the damage and the surroundings.