PREDICTED TRAFFIC AND ACTUAL TRAFFIC AFTER A ROAD WIDENING

by Wes Marshall

You already have a congested roadway, and the transportation planners predict even more traffic on that road in the near future. What do you do? For most of the last century, the answer was to increase capacity. In the short-term, this seemed to work. Time and time again, over the long-term, the actual amount of traffic after the capacity increase grew far more than expected. What seemed like an obvious solution to a congestion problem continued to disappoint. But why?

The reason for these failures lies with the principle of induced demand. Once capacity increases, not only do you get the originally predicted traffic growth, but you also facilitate some often unanticipated changes in travel behavior. First, existing road users might change the time of day when they travel; instead of leaving at 5 AM to beat traffic, the newly widened road entices them to leave for work with everyone else. Second, those traveling a different route might switch and drive along the newly widened option. Third, those previously using other modes such as transit, walking, bicycling, or even carpooling may now decide to drive or drive alone instead. Together, these unwanted behavior changes fall under what is termed the theory of triple convergence (also known as the ‘Iron Law of Congestion’). This latent demand induces more traffic than originally expected and saps the supposed improvement of the expected benefits.

The joke is that adding lanes to cure congestion is like loosening your belt to cure obesity. Empirical results over the last century – due to the principle of induced demand – have borne out that this issue is real and should always be accounted for when considering adding capacity as a solution to congestion.

References:

Downs, A. (2004). Still Stuck in Traffic. Washington DC, Brookings Institute.