A paper a couple of year's ago studied the 3 foot passing law in Maryland (the law that requires drivers to pass cyclists with at least 3 feet). It asks the question " Is the three-foot bicycle passing law working in Baltimore, Maryland?" I would not be surprised if the answer to that question is no, but this study doesn't make that case, even though it implies that it does.
For the study, five cyclists rode around Baltimore in 2011, logging over 10 hours of video recordings which were then analyzed to see how many time cyclists were passed in violation of the law. Surprise - drivers were breaking the law 17% of the time. This says that the law is not causing compliance to go to 100%, which is a pretty high standard. But without knowing the rate of close passing before the law went into effect, it can't tell us if the law worked or not. If 80% of passes were less than 3 feet beforehand, that would make the law pretty effective.
I don't think that's the case and I'd be surprised if the law changed much in that respect. Most drivers probably don't know the law and at the time of the study it had only been enforced twice - both times after a crash; but even after a crash the law has value - if it helps a cyclist to win a court case for example.
Here's another criticism of the study which is mostly concerned with its conclusion that driver compliance was 100% if there was a bike lane present.
Jim Titus has written about the Maryland law and its confusing number of exceptions, so Maryland seems like an odd place to study the 3 foot passing law on a national level. Efforts to improve it have failed.