Wide Highways and Nash Equilibrium
This article from City Observatory, an urban planning-focused think tank, focuses on the implications of expanding already wide highways. Specifically, it focuses on the city of Portland’s Rose Quarter Freeway Widening Program. This $500 million program is aimed at decreasing congestion and traffic jams on Portland’s highways by constructing new lanes. Initially, this may seem like a reasonable solution to the problem. After all, if traffic jams are the problem, adding more space seems like it should help. However, the author discusses something that they call the “fundamental law of traffic congestion.” This principle states that adding more capacity to roads encourages people who would have otherwise travelled another way to user those roads. This “latent demand” for travel over vacant roads fills the road to the extent that it is no less congested prior to its expansion. In this way, adding more roads maintains the same driver density while increasing the total number of drivers, which in turn can lead to more accidents and traffic jams.
When I read this article, I developed a mental model of driving as a game that is not under equilibrium. Consider some individual that is looking to drive to work. They have a regular route that does not require them to drive on the highway. They also can take another route that lets them use the highway. On this highway, their travel time is a function of the number of individuals on the road such that as the number of travelers increases, the time to travel does as well. This is because more people means more congestion, distraction delay, and accidents. In such a game, adding a lane to a highway may alter the function. However, as the highway provides a promise of higher speeds, more and more people will decide to take the highway over the alternative route until the highway speed function no longer yields a favorable result from them. In reality, all roads have their speed determined by functions of environmental factors and none have flat times. Since highways are some of the most prominent roads, those that expand attract new drivers quickly, and consequently, slow down quickly. An ideal driver will consider the conditions of all paths and choose the most favorable one. Real people seldom drive like this (at least not completely).