Dude, where are my cars?

Pacific Northwest sustainability think tank Sightline Institute’s Programs Director and blogger Clark Williams-Derry evaluated 2010 Census data and Total Vehicle Miles (TVM) data to show his region’s car usage is leveling off despite the traffic engineer led push for increased highway capacity. In short, Williams-Derry research contributes to the ever-growing list of work that point to the average American driving less.

Williams-Derry knew something was amiss when local news outlets were reporting that Vancouver’s Golden Ears Bridge was not meeting revenue projections, despite traffic engineers constant push to increase area highway capacity. So, Williams-Derry embarked on his own analysis. By coupling 2010 census data with TVM, he validated that be a long-term slow decrease in highway usage that predates the recession. Based on his methodology, Washington State alone experienced an 8% decline. Counties that encompass the greater Seattle Metro Area saw per capita TVM decline in the range of 7%-11%. Impressively, King County, which contains the City of Seattle, saw a 10% decline. Also, he examined Seattle’s bridge crossing data. He found that traffic increased in 2010, but it did not keep pace with population growth. In addition, he applied his methodology to Spokane and the results were flat over the last decade. Similarly, small counties in Eastern Washington have seen no net traffic growth since 1994.

If these declines are true, then why are engineers still advocating for larger lanes, bigger bridges, cloverleaf interchanges, and other related highway infrastructure? Williams-Derry maintains these experts base their methodology on an inadequate often-deployed equation called linear regression. For highway engineers, linear regression correlates population growth with a need for greater highway capacity. This model often times does not account for the growing sustainability trend, the urbanism movement, higher gas prices, the weak economy, and a host of other unknowns. Unfortunately, linear regression’s use is widespread and both the 1991 Washington State DOT forecasting manual and the Oregon DOT advocated on its behalf.

Now backed by his findings, Williams-Derry is encouraging engineers to adopt an improved process to evaluate the need for new capacity expansions. One way he is achieving this is by writing Dude, where are my cars?. This series is a thirty-four part featured blog on Sightline Daily and has spanned a time period of nineteen months. This worthy read summarizes his methodology and results, explains flawed traffic forecasting, and reports on news worthy items that validate his conclusion.

Dude, where are my cars?