For the most part, using your dash cam to record video on public streets and parking lots—where you’re almost always driving—isn’t breaking any privacy laws. That’s only for video, however. If your dash cam also records audio in your car (or has a second lens recording the inside of your car), you need to inform the people you’re driving with about the recording or it can be considered wiretapping. Bambauer lays out the types of wiretapping laws you’ll find from state to state:
Some states have “all party” laws, where each participant in a conversation or other setting must consent to the recording. So, if you are in one of those states, and you’re recording with your dash cam while your friend is in the passenger’s seat, you need her permission to make the recording. If you don’t obtain it, you could face a civil suit (assuming your friend gets mad enough to file one) or criminal prosecution. In “one party” states, any participant can consent to recording, so as long as you consent (which you likely do, since you’re running the dash cam), you’re safe from liability. And, if your dash cam is running while you’re not in the car, you might run into problems even in one party states, since it could record interactions to which you are not a party.
If you’re worried about it, you can either inform everyone who rides in your car (nothing else necessary), buy a model that doesn’t record audio, or disable the audio recording functionality.
Except as otherwise provided in this article or permitted by federal law, it shall be unlawful for any person to operate any motor vehicle on a highway with any sign, poster, colored or tinted film, sun-shading material, or other colored material on the windshield, front or rear side windows, or rear windows of such motor vehicle. This provision, however, shall not apply to any certificate or other paper required by law or permitted by the Superintendent to be placed on a motor vehicle’s windshield or window.
No person shall drive any motor vehicle with any sign, poster or other nontransparent material upon the front windshield, sidewings or side or rear windows of such vehicle which obstructs the driver’s clear view of the highway or any intersecting highway.
每个州的法律条款及具体用语都不一样，有的更含糊，有的更明确，据我所知并没有哪个州的法律专门提及「dashcam」。上面提到的律师 Christopher Jones 说，在 Virginia，「dash cams are legal as long as they are mounted in a way that does not obstruct the driver’s view of the highway through the windshield, front side windows, or rear window」；我也持同样的观点：只要不影响驾驶员视野就行。