The people who make the case for highways often present themselves as unbiased technicians, simply providing evidence to an audience subject to irrational bias.
But traffic forecasting is not a neutral, dispassionate exercise. It is subject to all sorts of incentives, beliefs, and assumptions that can skew the results in a particular direction.
Intentionally or not, forecasters frequently exaggerate predicted traffic volumes to make the case for building toll roads, according to industry consultant Robert Bain [PDF]. Bain has catalogued 21 ways in which forecasters manipulate data to make toll road financing look attractive [PDF]. Gaming numbers isn’t limited to toll roads — DOTs do it for taxpayer-funded projects too.
Here are a few tricks Bain says forecasters use on private projects to make highways seem like a good bet to investors:
1. Pick a time frame that suits you
Maybe looking at the last 10 years of traffic doesn’t make that great a case for widening a highway. Why not just pick a different time frame?
To justify its $850 million I-94 expansion project, Wisconsin DOT used traffic data from 1999 through 2010, leaving out two years. But traffic was flat on the road between 2009 and 2012, according to a Wisconsin PIRG analysis, which has pointed out the agency is a notoriously overoptimistic forecaster [PDF].