Roads to prosperity? Assessing the link between public capital and productivity

Does the positive correlation between infrastructure and productivity reflect causation? If so, in which direction? I find that when growth in roads (the largest component of infrastructure) changes, productivity growth changes disproportionately in U.S. industries with more vehicles. That vehicle-intensive industries benefit more from road-building suggests that roads are productive. At the margin, however, road investments do not appear unusually productive, Intuitively, the interstate system was highly productive, but a second one would not be. Road-building thus explains much of the productivity slowdown through a one-time, unrepeatable productivity boost in the 1950's and 1960's, (JEL E62, O47, R53).