&gt; <i>In practice, people can tell the design speed of a road and mostly follow that, no matter what the signs say. Defecting or not doesn&#x27;t matter very much.</i><p>It does, because what people can tell is <i>only</i> the design of the stretch of the road they&#x27;re at. They have no awareness of how it interacts with rest of the roads in the area. By ignoring the rules they make it significantly harder to optimize traffic flow globally. And <i>even if</i> the limits were wrong, it&#x27;s still better to have bad rules that are actually followed, because then people responsible can actually observe they&#x27;re bad and change them for the better.