1) Law enforcement is allowed to record a conversation without your knowledge or permission, so in that case only "1 party" needs to "consent." (Which in that case, would be law enforcement). In the State of California... that is.
1a) So quit using "the cops have cameras so I can record a conversation too" as justification.
2) The laws regarding when a private citizen can and cannot record are based on reasonable expectations of privacy. In general, conversations in public do not meet that criteria and you are free to record. There very well could be some conversations, even in public, where that could be argued against in court and you could lose.
2a) You can record your conversation with law enforcement in the State of California because the law enforcement Officer has, legally, no expectation of privacy, therefor it only requires "one party" to consent (which in this case would be YOU) and thus, the recording is okay. There are some states where this is not legal, but in California it absolutely is. However, if you were to, say, call a local Police Department and unknowingly record that conversation -- that could be used as a form of wire-tapping or unlawful recording. That is a conversation which would REASONABLY be considered private. If not, every time a citizen called the cops we'd be allowed -- and it'd be expected -- that we would go tell everyone in the world all of the confidential shit we learned or were told. Obviously that's not the case.
3) In most cases, what you record on you GoPro while riding on public roads -- including acts of road range or conversations through open windows to drivers etc, etc, etc. is not going to be considered reasonably private and thus, free to record at any time.
