So editorializing for a moment. In the US at least speed limits are rarely enforced to the letter. Everyone, including most police, know that a "25" limit really means 30, or 35 if you are pushing your luck. You will rarely, if ever, get a ticket for going 30 in a 25 posted area. Or 35 in a 30 posted area. The "overage" is about 5mph, although perhaps a little more at faster speeds (its certainly not proportional though).
This differs from other countries I'm sure. For example, in the UK limits are more strictly enforced. However, in that case the limits are generally higher than in the US (most minor roads in the UK are 30mph, 25mph is very rare). So the actual effect, in both the UK and the US, is most people drive at about the same speeds (30mph on minor roads, 40mph on major urban roads etc).
BUT, along comes technology to enforce speed limits. In the case of Tesla, its the stop light beta for TACC, but in theory any car that can read speed limit signs could limit even manual driving to not exceed the legal speed. In the US, that's going to cause chaos. Basically it will slow traffic down, cause congestion, and have (imho) no measurable effect on accident statistics.
And at that point we are at an impasse in the US. You can't raise the limits ... even ignoring the cost of re-signing vast stretches of highway, people here would just continue driving above the (higher) posted limits anyway. But you dont want people driving at the existing posted limits. AND you dont want to give waiver to self-driving cars to "break the law".
Ultimately, as others have noted, this is the first tiny step down a long road that has different restrictions for human vs self-driving cars. And what a brave new world THAT will be.
