Earlier quoted context omitted.
Safety and posted speed limits are often not super compatible; the safest thing, in most road conditions, is to be driving relatively close to the speed of the surrounding traffic, even if that means exceeding the posted speed limit. (in other words, speed itself is not dangerous except in certain situations -- like sharp curves or wet/snowy/icy roads -- speed differential is dangerous)
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…
So, there are three speeds we need to be concerned with here:
1. Design speed -- this is the speed anticipated and planned for by the engineers who designed the road. It's visible in features like curves, merging areas and so on.
2. Operating speed -- this is the actual speed of traffic on the road once built, and typically is measured as the 85th percentile of observed traffic (i.e., the speed such that 85% of traffic travels at or below that speed).
3. Speed limit -- this is the posted maximum above which vehicles can be stopped and ticketed by an enforcement officer.
In real-world scenarios both the design speed and the operating speed are often higher than the speed limit, because speed limits are often more strongly influenced -- and always downward, when they are influenced in this way -- by factors other than safety (i.e., politics and revenue).
Which is a problem. Good road design anticipates roughly what the operating speed will be, and attempts to match that in the design. Setting a limit which differs significantly from the operating and design speeds is only good for politics ("we're making you safer by slowing down the traffic") and revenue (more tickets issued for speeding); it has no relation to actually-safer roads.
And in general, yes, speed differential is more commonly a danger than simple raw speed; the intuitive explanation is that every event of one vehicle passing another creates an opportunity for a collision, and moving at a speed significantly different from all other traffic (regardless of whether faster or slower) increases the number of passing events which necessarily increases the chance of a collision.
Thus, regardless of posted limits, it is safer to match your speed to that of surrounding traffic. This is not a case of "everyone should defect because everyone else does". It's a case of "the people who posted the speed limit defected, and you shouldn't follow their example".