There seems to be an uptick in freeway news lately, so I put together a highway edition of my semi-daily news roundup last week, including the updates on the apparent death of the Columbia River Crossing, the 1960s-style "Opportunity Corridor" proposed in Cleveland, and a broader anti-freeway movement taking hold in the Midwest. The self-defeating nature of highway construction and expansion is noted in each of these articles, as is the physical scarring of communities that usually accompanies these projects. Lost in the discussions of induced demand (focused on the highway itself) and community displacement and segregation (focused on local residents and businesses) is the intersection of these two issues, local traffic.

Induced demand in the context of vehicle capacity simply means that building more space for cars encourages more people to use them. If I live in the suburbs and the city widens the freeway into downtown I might take a few extra trips into the city every month or decide that taking a job in the city is now feasible. The increase in average highway travel speeds also brings more distant suburbs and exurbs within driving distance, which encourages more development at the current sprawl boundary and beyond. These and other effects lead to more cars using the highway until the excess capacity is soaked up and traffic is just as bad as it ever was. According to most studies, about 80-90% of the excess capacity is soaked up within just five years.

This makes for a strong argument against highway expansion, but it ignores the impacts on local streets, which are far more severe. The problem here is obvious: unless 100% of the new highway users are bypass traffic--none of them using the highway to get into the city itself--local roads have to deal with a huge influx of additional vehicles. Many of those vehicles aren't bypass traffic, of course, so local streets (and their residents) are burdened with their presence and the congestion they bring. And while federal- or state-owned highways can sometimes afford to increase their capacity, local roads usually can't. Besides being a poor choice from an return-on-investment and livability perspective, widening local streets to cope with the increase in vehicles is usually physically impossible--city centers are already built out and, thankfully, few people support tearing down homes and businesses just to expand local roads.

In an effort to visualize this phenomenon, I've made this simple graph, which shows a schematic of capacity vs total traffic on highways and local roads before and after highway expansion: