Introduction: A bridge over the St. Croix

In 2017, a new bridge opened connecting Minnesota and Wisconsin across the St. Croix River. The cable-and-concrete structure spanned the gorge of the scenic waterway, the product of hundreds of millions of dollars, and years of federal legislative effort from delegations in both states (including then Congressman Sean Duffy, now Secretary of the US Department of Transportation). An aged, flood-prone trestle bridge nearby was retired into service as a pedestrian and bicycle crossing.  

What was the impact of this significant investment? Traditional measurements of transportation performance evaluate roadway segments, and would celebrate the increased volume, and especially the speed, along the bridge and its connected roadways. It was now much easier for people to drive cars and trucks across the river. And in the first few years after the bridge opened, people did travel more on the Minnesota state highway segment just west of the bridge—in fact daily traffic was almost 40% higher even in the teeth of the COVID-19 pandemic. So, the bridge was clearly being used. But where were people and freight coming from, or going to? Would these trips have happened without the bridge investment? What other consequences might arise from this initial increase in speed and capacity? These questions fall squarely outside of the benefit-cost analysis used to justify the years and dollars of effort. 

But people travel from origins to reach destinations, and the question of which trips were made easier, can be answered. One way is through a concept called Accessibility, which captures how easy it is for people to reach desired destinations. In this study, we explore the application of Accessibility to understand what it is state and local Departments of Transportation (DOTs) are providing for their residents, and their money.