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New Roads Can Make Traffic Worse

Link: https://www.wired.com/2014/06/wuwt-traffic-induced-demand/

This article dives into the mystery of traffic congestion. How is it that when new roads are added traffic doesn’t decrease? This is a puzzle that traffic engineers have wrestled with for many years – going as far back as the 1960s; they have been unable to draw any conclusions until recently because of a lack of sufficient data.

Two economists looked at traffic congestion data and new roads that had been built in various cities across the U.S. from 1980-2000. They discovered an interesting piece of information: the percentage of road capacity that increased due to the addition of new roads had a direct positive correlation with the percentage of miles driven due to new drivers that used them. The economists’ interpretation for this correlation is “the fundamental law of road congestion”, which says that new roads will attract new drivers and lead to unvarying traffic congestion.

In lecture we discussed Braess Paradox: adding a strategy to a game can actually exacerbate the situation for the players involved. This was demonstrated with an example in class that involved four cities (nodes) and four roads, shown in the figure below:

At equilibrium, all travel times are 65 minutes. If we add a new road from node C to node D that has a travel time of zero, we notice that the travel time at equilibrium actually increases from 65 to 80 minutes. This shows how adding resources can be disadvantageous for the drivers.

The article describes congestion pricing as a potential solution to this problem. If drivers are charged for using certain roads during rush hour, it will motivate them to find other means of getting to their destinations, such as public transportation; however, this solution isn’t popular because people don’t want to pay for a good – in this case a road – that they have always had for free.

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