I'm not a lawyer but I know more about this subject than I'd like.
A person in a vehicular accident has no right to the video footage from another citizen's dash camera unless it's introduced as evidence. However, if you are recording a personal interaction without informing the person they are being recorded things could be trickier. The police cannot force you to share video footage without a subpoena or a search warrant, and that usually only happens if they believe the footage contains evidence of a crime. Most vehicle accidents are not "criminal", merely illegal or infractions. Under extreme circumstances the police are allowed to confiscate a camera if they have a reasonable belief that it contains evidence of a crime and that will be destroyed - this is similar to the leeway given law enforcement under a Terry Stop. But even if law enforcement confiscates a camera the footage cannot be viewed until a search warrant is obtained.
A citizen can voluntarily turn over video footage to law enforcement but it's almost impossible for police to issue a ticket or citation based on the footage alone. However, if you submit footage of expired tags or a broken taillight and an officer visits the offender and they witness an old tag or broken light they can then issue a ticket or citation. Speeding or reckless driving footage will only result in a written warning (at most). The written warning can be used to build a stronger case or impose harsher penalties if in the future another instance of the same infraction is witnessed by law enforcement.
I have issues with people that feel it's their duty submit selective footage only of the infractions that upset them, that seems vindictive. But if someone is submitting ALL footage of infractions (no turn signal, failure to come to a complete stop, etc) then I could understand why they'd also submit footage of something that directly affected them. But if someone is reporting all infractions their camera captures that would practically be a full time job. I'd think a person like that is a little crazy, and so would law enforcement.