Do you have a citation for that? It sounds plausible but I bet the real situation is more complicated. The highway speed limit may be 70mph, say, but from an e

Do you have a citation for that? It sounds plausible but I bet the real situation is more complicated. The highway speed limit may be 70mph, say, but from an emissions point of view, for most cars the ideal speed is more like 60mph. From a safety point of view, the ideal speed will depend largely on traffic conditions. 85mph might be fine in light traffic on a straight road in clear weather in daylight, less so in other conditions. That seems like a good argument for lowballing the speed limit (assuming that you're correct about that).