I hear that car "dashcams" start when they receive input voltage and record in a circular queue so there's nothing to do. Maybe one of those would work for you. You'd need a speed-sensitive 12v power supply, but if you can fashion that, you are set.

Of course, I have to tell you some bad news. You're preparing for your last mishap, not your next one. It's good to be prepared, but your preparation should be in accordance with your risk.