[mannykannot]

In the places where the speed of road transport has most influence on productivity, the speed limits are largely moot because of traffic congestion at the times when most commuting happens. In places and at times when congestion is not a problem, speed limits rarely have much effect on productivity.

[v01dlight]

I think the point about productivity is less about people's 'burbs to downtown commute and more about say, the trucking industry. Trucks drive at or near the speed limit for much of their long highway routes, and over those distances, a slight difference in average speed can mean a big difference in arrival time, and throughput of the whole system.