Just tell him your car has sophisticated on-board telematics systems that track your car’s position and motion ten times every second. The computer will not lie when it has recorded that you were at a standstill and suddenly moved backward while the gearshift was still in D. Then ask him to write down his lie for the record, just so his insurance company knows exactly what kind of customer they are dealing with.
But in reality, the insurance company almost certainly won’t care what happened. They’ve got more important work to do than send CSI State Farm out for a couple of bumper replacements.
In cars, the data recorder is part of the airbag system. It constantly overwrites the memory until the bags deploy. It is designed to provide only a few seconds of data immediately before airbag deployment so it can be determined whether the airbag deployment was appropriate. That the data can be used to demonstrate handy details regarding speed, braking, etc. immediately before the collision is merely a side-benefit. What it is NOT is akin to a flight data recorder on an aircraft. In addition to the constant purge, the memory can be purged by turning the key in the ignition several times.
As for the OP: if the only evidence is going to be word vs. word, absent multiple similar events related to the car or driver to suggest a history of contested and similar events, you just screwed.
Not a bad idea, the field expedient method is to see if the lamps have ‘burned out.’ The theory is the filaments in the bulbs are hot and brittle, and break when subjected to a moderate+ collision. Conclusion: if bulbs are dead, they were illuminated upon impact. It’s not fooproof, but it’s another bit of evidence.
Now I have to go look up light filaments in accidents. I remember reading a newspaper article many years ago (Dallas Morning News “Crash of 50 Sugar Kilo” IIRC) that indicated just the opposite, that stretched filaments are an indicator the light was on and that they can also estimate G-force of impact by the way the filament stretched.
And just to clarify as I really only got half the thought out:
Both lit and unlit bulbs will break in an accident, but unlit bulbs fracture without stretching, the filament shatters. In contrast, lit bulbs are deformed and stretched to their breaking point, much like bending a wire coat hanger back and forth until it breaks.
So if someone was backing into you and hit hard enough to knock out their back-up lights, you could look at the filament and tell if it was lit or not at the time of accident by the way it broke.
In an accident in which the two parties have completely different stories (he had the red light. No he did!) and there is no evidence or witnesses to sway it to one side or the other then I would just put in both accounts and let [del]God[/del] the insurance companies sort it out.