"And if everyone complied, there would never be another collision."
I'd dispute that. When a flat-fronted vehicle passes me at around 3 feet, at speed, on a windy day, it can take a lot of skill to avoid being pulled towards it by the suction it creates (this also causes caravans to "snake" on motorways - I'm not imagining it).
It would be entirely possible for a cyclist to be passed at 3 feet, but then be sucked under the wheels of a large vehicle. But it'd be alright because the driver had left a 3-foot gap.....?