Are warning signs irrelevant?

Who is the better judge of appropriate speed – you and me at the time and the place, or limits fixed by absent regulators? Once I was caught in a cloudburst on the M4. Everyone had slowed to a crawl. Twenty minutes and ten miles later, after the storm had cleared and we had long since picked up speed, a gantry sign warned: SPRAY! SLOW DOWN! That's how irrelevant signs are to the needs of the moment.