THE logic behind congestion pricing—tolling roads to maintain free-flowing traffic conditions—is pretty straightforward. When a driver enters a road space, he receives some benefit (the mobility provided by the road) and faces some cost (the expense of driving, including time cost). What he doesn’t have to bear is the cost of the congestion he creates by driving, which is borne by every other driver on the road. That’s a negative externality; it means that too many drivers will use a road and cause it to become congested unless that cost is somehow internalised—as through a congestion toll.

Now, here’s the dramatic twist: a paper which provides evidence of other negative externalities from congestion by studying traffic created by…toll booths:

This paper provides evidence of the significant negative health externalities of traffic congestion. We exploit the introduction of electronic toll collection, or E-ZPass, which greatly reduced traffic congestion and emissions from motor vehicles in the vicinity of highway toll plazas. Specifically, we compare infants born to mothers living near toll plazas to infants born to mothers living near busy roadways but away from toll plazas with the idea that mothers living away from toll plazas did not experience significant reductions in local traffic congestion. We also examine differences in the health of infants born to the same mother, but who differ in terms of whether or not they were “exposed” to E-ZPass. We find that reductions in traffic congestion generated by E-ZPass reduced the incidence of prematurity and low birth weight among mothers within 2km of a toll plaza by 10.8% and 11.8% respectively. Estimates from mother fixed effects models are very similar. There were no immediate changes in the characteristics of mothers or in housing prices in the vicinity of toll plazas that could explain these changes, and the results are robust to many changes in specification. The results suggest that traffic congestion is a significant contributor to poor health in affected infants. Estimates of the costs of traffic congestion should account for these important health externalities.

Congestion in America is pretty costly even without taking these effects into account. And in most cases, the back-ups aren’t due to toll booths, but to the fact that roads are free to use.