Maine just passed that law about a year ago, and although it
is hard to see a provable effect, I think overall it's worthwhile. It's certainly an incremental thing. Sure not everyone has heard about it, and not everyone who's heard about it likes it or cares, but then many people have and do. I
think I've noticed a slight increase in safe passing, even in the number of people who pull clear over into the opposing lane to give me about 10' of room, which feels like kind of a mixed blessing! And I do agree that it's legally useful to have a number, although obviously that can't be proven in court with a ruler.
I had a close passing incident with a box truck last spring, and reported it to the police with the license number. The cop of course was pretty skeptical that there was any point in attempting to issue a ticket without other witnesses, but he did agree to talk to the registrant about it. I never got a followup, but I feel that having the law enabled me to make a more specific complaint. I was able to speak to the driver at the next light, and he obviously hadn't heard about the law (that he was letting on) and seemed incredulous that he had any duty towards me at all since I was not "in the path" (which was a debris-strewn shoulder). So yes, there will always be jerks.
FYI, our
excellent state bicycle coalition was the prime mover behind the new law, working with bike-friendly state legislators to write and lobby for the bill. I think this is a good model for getting the legislation done anywhere.
FWIW, our law included a provision that motorists may cross a double yellow line in order to afford the 3' distance, the theory being that this would decrease attempts to squeeze through. Maybe this is why I get more people giving me much more room than they need to, because they hear "allowed to cross the double-yellow" as "allowed to pull into the opposing lane"!