Study Finds Traffic Forecasts Consistently Overestimating Congestion

Traffic forecasts often deliver influential input to the debate over whether or not to build new infrastructure to meet growing demand. The forecasts are important because they are used to calculate key performance indicators such as accident reductions and economic rate of return. As a result, they can play a decisive role in arguing for or against individual projects.

In a new article published in the scientific journal Transport Policy, two researchers from Aalborg University in Denmark and the Norwegian University of Life Sciences in Norway have compared the predicted and the actual traffic levels in business-as-usual scenarios used for Danish and English road projects. Their research shows that the traffic forecasts have predicted too high a growth rate in 7 out of 10 projects.

In the abstract, the researchers claim that "the main implication for planning practice is that the severity of future congestion problems is systematically overestimated. As a consequence, impact appraisals of road construction as a means of congestion relief appear overly beneficial." While it has not been possible to pinpoint the exact causes for the observed inaccuracy, the researchers argue that the most likely cause is that traffic is often assumed to continue growing regardless of available capacity. In reality, they claim, some people will find alternatives when congestion levels get too high, possibly choosing public transportation, moving closer to their workplace or finding a new job with a shorter commute time.