I think we're overextending the point a little, and apologies if my point did not get across well.
What I wanted to illustrate was that those kind of speeds, in heavy vehicles, in cities, on surface streets, are negligent - spiritually. The sheer amount of damage that can be inflicted, in the blink of an eye, by a moment of inattention, is beyond measure (literally, that 67k number looks goofy, unreal, but it's cold hard physics). Our traffic laws are wrong, our streets are wrong. We don't tolerate that anywhere else. On a factory floor with dangerous machinery there are higher standards of safety than our public realm. This news article is the perfect illustration. Good chance the driver fell asleep, or was distracted in some way (totally normal human traits, btw, and does not make somebody a bad person). Yet the lawyer claims, that at 75mph in a 5000 pound car, this is basically just worth a speeding ticket. That's an enormous disconnect. That delta between responsibility and accountability is totally totally out of wack for drivers. An enormous amount of responsivblity (67k kg*m/s worth of it), and very little accountability ($100 speeding ticket).
And yes, at slow speeds, there's just a lot more slack in the system. The driver of a 5mph tractor trailer has a lot of time to react, so does every one around them, stopping distances are exponentially lower etc... It doesn't change the fact that it will level a house, and so we should have extremely high standards.