Re: I see some potential in this...
« Surely it would be best if the road could tell your car what the speed limit was »
That would be automatic sign recognition, coupled with a good navigation database that already contains speed limits. Both exist already and are actually handy.
The magnificent know-it-alls in here will tell you they can just look out the windscreen and read the signs (which is exactly what the car itself does), and yes, you can. If you can also keep in memory whether the speed limit on this particular stretch of road was X or Y or maybe Z while also busy driving the car, spotting conflicting traffic, navigating, looking out for and anticipating hazards, then you do not need this. On the other hand those of us who are merely human do appreciate the extra help.
As for keeping within the speed limit, cruise control helps lots. If it is adaptive cruise control it will also make you far less likely to tailgate the slightly slower moving vehicle in front of you, and it also reacts quicker than a human in an emergency breaking situation.
Again, the above is just my own experience, but at least it IS experience, rather than trying to be a smartarse making armchair comments as to why hundreds of experienced engineers are just "idiots".