You can always video, since there is no reasonable privacy expectation for passengers in another person's car.  For audio, since they could be speaking on a phone, to another passenger, etc.--i.e. not only directly to you--you need to inform them *before* they get in the car that you will be recording them, so that they have the chance to decline your services before you start recording. (If they use you and get in your car with awareness that you will record, they can be said to have consented to it.) If you record them without consent, you may be violating the same law as is broken by wiretapping; and if you wait to tell them until you are driving, they can't effectively consent: because they are a "captive audience" at that time, their consent will be held to be coerced and not valid. In addition to orally telling them before they get int, have a prominent notice or sign where passengers sit reinforcing the recording.