My God, this is entirely, entirely, wrong. When you speed, you're JUST as culpable of killing someone whether you actually kill them or not. This is an old ethics issue: you can imagine two scenarios, both of which you do exactly the same actions: speeding down a road. The difference is that in one of them, a pedestrian doesn't see you bearing down on him, and you kill him. The pedestrian may or may not be there, but in one scenario, you're a monster, and in the other, you're just a speeder - even though your actions are identical in each case!
The right answer is that you're a monster in BOTH scenarios, and you should never fucking speed.
I think you missed the entire point of the discussion.
Where speed limits are set arbitrarily or cannot be set at a consistently appropriate level, there is no moral consequence of exceeding them.
Thought experiment: A geriatric, possible senile driver runs his car into a traffic barrier at 5mph on a busy road. In my infinite wisdom I declare that 5mph is clearly too fast for geriatric, possible senile old men and set the speed limit to 3mph. Is it then immoral for you to exceed 3mph on this road?
How about 4mph? 5? Etc. up to arbitrary speed limit.
You're conflating driving at a certain speed with driving at a speed which is unsafe, which is purely a question of degree. 10mph may be marginally more dangerous than 5, but that doesn't make it immoral to drive at 10mph.
Therefore, if you drive at a speed which is unsafe and likely to result in your scenario above, you are morally responsible for your actions. But there is nothing immoral per se about an expert driver, in clear conditions, exceeding a speed limit while maintaining control of the vehicle and being aware of their surroundings.
It's a waste of time. My suspicion is that we're arguing with a bunch of folks who spend a lot of time riding buses, and/or who don't even hold drivers' licenses.
The right answer is that you're a monster in BOTH scenarios, and you should never fucking speed.