You like the analogy because a speed limit of 50 would result in a decrease of fatalities due to car accidents? I guess that's what he meant with "cost to society" since IIRC more fatalities result in a higher gross national product.
The only increase in GDP that can be attributed to roadway accidents is due to the spending of medical care, which often times is loaned or from insurance, which is not a stable and reliable way to increase GDP over the long term. Overall, fatalities decrease GDP.
That's probably what he meant but it's not necessarily how you have to read the quote in the parent comment. I'm still not convinced that a speed limit of 50 would by a bad thing but maybe that's a cultural thing. My memories of my economics class are fading so I cannot counter your argument about the stability of the increase.