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"Windshield Witness" dash cam is designed specifically for what you are talking about. They do NOT record GPS speed (or any kind of speed reference) because this can be used against you very easily. You may not have been speeding at the moment of the accident, but if they find you were speeding prior to the accident they will state you arrived at the accident location "because" you were previously speeding. If you had not been speeding earlier you would have arrived at the accident location AFTER the other person had passed. Interesting logic approach but quite appropriate.
The video is continuously date and time stamped and it uses a "g-force" sensor that will lock the memory card data in the event of an accident. To protect yourself or to prove your innocence all you need is the visual proof - no sound, no speed, no GPS information - the picture/video will tell the entire story.