Filming a traffic infraction in the US can result in a traffic ticket for the offending driver?
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Accepted answer
Since you are asking here on Travel Stack Exchange, I guess you plan to do that in a country other than your own.
Many countries have traffic regulations which prohibit drivers from playing with their phone while driving. Submitting the video as evidence would implicitly admit a traffic violation of your own.
Some countries have privacy laws which limit making videos of other people without their consent. These laws can be complex and come with many exceptions. The safe thing is to ask politely before you make pictures where other people are recognizable.
A permanently installed dashboard camera might violate these laws, not because it records a traffic violation but because it records the time before and after.