That's the point though, there's already legislation in place called the Highway Code. It states that motorists should give a cyclist as much room as they would give a car. Now most motorists don't obey that but at the same time, the vast majority overtake safely. And how exactly are you going to measure 3ft when a car is overtaking? Are you then going to phone the police and say "car reg xxxx overtook me with only 2' 4" space"? No, so it's totally pointless.
Turn it the other way round – suppose you're filtering through stationary traffic, bars inches from wing mirrors and someone says "sorry mate, you need to give 3ft space". You can't have it all your way.
The time and effort would be better spent encouraging more people to ride bikes. The more cyclists on the roads, the safer the roads become. They don't become safer by more and more unenforceable laws that make the motorist feel ever more alienated and discriminated against.
End of rant – I'm off for a ride…