Traffic forecasts play an important role in the planning and approval of new road and rail projects. They are important for determining both the expected economic and environmental project impacts, and therefore often end up being a highly debated item in the planning process.

In a new article published in Transport Reviews, two researchers from Aalborg University in Denmark show that decision makers should probably take the forecasts with a pinch of salt. After reviewing the largest studies that compare forecasts with actual demand, they conclude that "it is clear that demand forecast inaccuracy is problematic for all project types." The studies included in the review examined projects on all six continents from the 1960s to 2012.

The review confirms earlier results from individual studies that have shown road traffic to be underestimated on average and rail patronage to be overestimated on average. According to the researchers, both tendencies are problematic as planners tend to overestimate the congestion relief from new roads as well as the attraction value of new rail projects. However, even if forecasts were unbiased on average, the article argues that decision makers should be careful with relying too heavily on forecasts. Traffic on two projects could easily be 30% below the forecast for one and 30% above the forecast for another. In such a case there would be no bias, but the general lack of precision makes it problematic to assess individual projects.

The researchers argue that better monitoring is a first step in improving the usefulness of traffic forecasts. "It ought to be a key priority for funding institutions to implement better monitoring of project impacts, develop standardised archival procedures and make the data publically available, in order to facilitate more detailed studies of why travel demand forecasts are still often highly inaccurate."