I read on http://allucam.com/legality-dash-cam/:
Dash cameras obviously can make a visual record of the road ahead, but under state law, they must also be capable of recording and saving audio data. Because this requirement raises privacy concerns, you must post a notice in a visible place in your car that notifies passengers that their conversations are being recorded. The owner or lessee of the vehicle can turn the device off at any time. The information stored on the recorder belongs to the owner as a matter of law.
This state law is V C Section 26708 Material Obstructing or Reducing Driver x2019 s View:
(B) A vehicle equipped with a video event recorder shall have a notice posted in a visible location which states that a passenger's conversation may be recorded.
What place(s) in the car is regarded as a visible place for all passengers, how to assess whether the font is large enough, and how about visually impaired passenger?