I was under the impression that there was case law supporting it, where the AASHTO and FHWA standards were used as a defense for someone taking the lane under 14 feet wide? I don't have any links to such case law, though, and I'm not sure if any motorists have had judgments against them for not passing in the oncoming lane for a lane less than 14 feet wide.
Which, case law is just as good as law in the jurisdictions of the courts that made such case law.