I bought a dash cam after I was T-boned at a major intersection. Two witnesses stayed until the police arrived. Witness confirmed I had the green light. "Lady" that hit me didn't appear to speak English, didn't have a Driver's License or insurance either.
The dash cam I bought clamps over my inside rear-view mirror. One camera facing forward and the second camera facing rear. Forward camera can be easy rotated to record any interactions between the Driver (me) and anyone standing outside the driver's door.
I've recorded a real nice video of me stopped at an intersection, a large dump truck turning too close my way for my comfort and me backing my truck up to put my hitch through a Cadillac's front end.
My dash cam records with a time and date stamp. I personally chose a unit that DID NOT record my vehicles speed too. IF you were speeding and involved in an accident, your dash cam recording could be used against you. AND IF you erashed the incriminating evidence, you could be arrest for doing so.