Here is a good rule of thumb. Three feet is about the swing of a car door. If you can park next to another car and your passenger can get out without denting the car next to you that's three feet.

I commute by bike daily and 100s of cars a week manage to pass me safely. It is not that hard and has been common sense for 99% of all drivers who pass me even before this law. It is that one super-sized SUV who passes me with mere inches to spare while going 50 in a 35 zone.