Our analysis of TTI’s data shows that building new and wider roads has had little long-term impact on road congestion, and that such roads appear to actually generate additional traffic. In order to control for population growth, we looked at the amount of highway space per resident each metro area has added since 1982. We split the 68 metro areas included in TTI’s study into three groups and compared the congestion rates for the high and low group. The high road-building areas increased road capacity per person by 28%, while the low-road building areas actually decreased road capacity per person by 11%. Despite this wide discrepancy, the rush-hour congestion profiles as measured by the Travel Rate Index for each group are almost identical. Interestingly, the high road building areas show slightly higher congestion levels than the low road-building areas throughout the period.

One explanation for this outcome is that new and wider roads tend to generate new traffic. This phenomenon, known as ‘induced travel’, occurs when road capacity is expanded and drivers flock to the new facility hoping to save time. The new roadways also tend to draw people who would otherwise avoid congested conditions or take alternative modes to their destinations. In the long run, this encourages additional development nearby, and that leads to even more traffic.

Our rigorous analysis of TTI’s data¹ confirms this relationship. In the metro areas studied, a 10% increase in the size of the highway network has been associated with a 5.3% increase in the amount of driving. In other words, half of the new highway capacity has been filled with driving that would not have occurred if the road space had not been added. This is consistent with previous research on induced travel, including an FHWA sponsored study which found that when additional road capacity provides a 10% improvement in travel time, driving increases by 5%².