As a whitewater kayaker in Texas, I can concur with this saving us. But the police don't often know the federal law and will still harass or arrest you.
Also, federal law won't necessarily save you in court, unless you have the money and time to escalate. I took a traffic fine to court once, showed the judge copies of the federal regulation and photos of the area proving that the signage was invalidly placed, and he upheld the fine anyway--he literally said "I don't care."
