Re: 2 words: black box
You can't be hit from behind and propelled into the car in front and be at fault for that.
If you have stopped at (say) a red light for what is likely to be more than a second or two, and have not engaged your handbrake, then you are at least partly to blame if the handbrake would have held your car short of impacting the one in front. (The footbrake is ineffective because the jolt of the impact from behind is likely to knopck your foot off the brake).
Note for US readers: I'm assuming manual transmission "stick shift" in neutral and UK standard controls.
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