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.
I think the "cost to society" would be more directly attributed to the increased time of travel for people and goods traveling on roadways.
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.
> "cost to society" would be more directly attributed to the increased time of travel
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.
"It's like the FBI demanding that no automobiles drive above 50 mph, so they can more easily pursue getaway cars. It might or might not work -- but, regardless, the cost to society of the resulting slowdown would be enormous."