You never think you need a dash cam until you get into an accident, or see something really cool through your front windshield, so you really should consider setting one up before you miss something important.
Today on Amazon, you can get Aukey’s well-reviewed 1080p dash cam (with a Sony sensor!) for just $56, or Vantrue’s dual-lens N2 for $110. I recently demoed the N2 Pro (which has a 1080p rear-facing lens, vs. the N2's 720p), and while the rear lens definitely won’t help you in every lighting situation, it could provide some extra context in the event of an accident.