I think it’s because 70km/h is not “close to the speed limit” and also because there are countless reasons why you cannot drive a bicycle or a horse carriage on a highway besides the speed you travel at.
What reasons other than speed are there for keeping bicycles off fast roads? Motorcycle can be very nearly just as small and vulnerable as bicycles, but they can keep pace with cars and consequently they're allowed on every road.
Same reasons why you cannot drive small motorcycles on highways here in Italy, even if they can easily reach 110km/h they are not allowed on even second tier highways.
Maybe it's different in Italy, but in America the restrictions on bicycles really seem to be about top speed, not acceleration.
Overall the reasons why you cannot drive bicycles on highways are many, but when you put them all together it mostly fall under one term: safety.
What I wanted to say on my original comment is that trying to put an autonomous driving car in a urban situation is not about checking the speed limit from a road sign, looking at a ground stop or other road signals. If you want a safe autonomous car in an urban center it must be aware of the context much more than “this is a stop sign” or “speed limit is 50km/h”, that’s why I praise Mercedes for trying to solve the solvable problems first and then move to more complex ones.