Since the traffic simulation in LA Noire basically ignores you, NPC cars tend to hit you or pull directly in front of you when you would have the right-of-way. The only reliable way of avoiding this seems to be to use the police siren, which causes NPC cars to pull over.
Is there any disadvantage to using the police siren whenever you're driving? I know you can have your partner drive, but you need to drive around to find various street crime side missions.