Earlier quoted context omitted.
You're basically saying that everyone should defect because everyone else does. Which may well be true for a single human driver, but is probably not how autonomous cars as a whole should be programmed. Nor does it support the claim that "speed itself is not dangerous". If everyone were driving slower, everyone would be safer (which we may actually be able to achieve once a sufficient number of law-abiding robot cars…
>If everyone were driving slower, everyone would be safer But that's always true. All the way down to 10mph speed limits. The job of speed limits is not maximum safety at the cost of everything else. 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't matter very much.
It does, because what people can tell is only the design of the stretch of the road they'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 even if the limits were wrong, it's still better to have bad rules that are actually followed, because then people responsible can actually observe they're bad and change them for the better.