All I'm trying to do is point out that speed is not necessarily linked to safety. Would the Germans have no upper limit on certain roads if it weren't safe to do so? So why would it automatically be unsafe to do the same speed on an identical road which has a limit?
Because all other road users
might be expecting you to be travelling at or around the posted limit, not twice as fast - assuming the majority of traffic on that road sticks to the limit, which is by no means a foregone conclusion. Maybe everyone does 100mph there. Anyway, if you're travelling much faster than the average motorist on that road, then other users are more likely to misjudge your speed and get in your way, especially at night when it's more difficult to judge how quickly a pair of headlights are approaching. Essentially, though, in this case it can be argued that it's the speed limit that makes the high speed dangerous (and even more so a very low speed), not really the speed of the vehicle in itself.
Generally speaking, the safest speed to be doing on any road is the same speed as the majority of traffic, regardless of what that speed is in relation to the speed limit, even if the majority are doing 250mph on slicks in a blizzard. It follows that the safest speed on a bicycle, assuming it was highly visible (bright reflective colours, lights, blah blah) to make up for its smaller cross section, on the same road in the same blizzard would also be 250mph. Good luck to the rider... most I ever got on a bicycle was around 60.
However, that's leaning more towards being pedantic, because I agree with your underlying point, and the point of your fast driving friend, that driving at high speed is not automatically unsafe. It's just a bit less safe than doing the same speed as everyone else because it gives others an excuse to get in your way.