Braess’s Paradox in Real Life
Our intuitive sense of network traffic says that if we add more capacity from Point A to Point B, the travel time between the two points should decrease. However, this isn’t always that case, as we demonstrated in class with Braess’s Paradox, which is also explained succinctly on Wikipedia.
However intriguing as it is, Braess’s Paradox is very theoretical. I wanted to see if there were many real-world examples that had been studied. I read a 1996 paper by Phillip Goodwin on Induced Traffic, which finds that, on average, when additional road capacity is added, additional traffic on that road is induced, typically around 20% of pre-expansion traffic. This phenomenon is not new; instances of induced traffic have been recorded as early as 1938, when the use of the automobile was first becoming widespread.
Although counterintuitive, induced traffic is commonplace. The paper cited above analyzes 151 instances of “improved” roads in the UK. Of the roads in the study, the effects of induced traffic were most pronounced on rural roads and newly constructed roads that bypass congested areas.
Even more interestingly, examples have also been recorded of induced demand functioning in reverse. During the 1989 Loma Prieta Earthquake, the Embarcadero freeway in San Fransisco was damaged to the point where it was no longer safe to travel on. The city faced a choice of whether to replace the freeway with an expensive elevated structure, or simply to demolish the ruined highway, with a net reduction in capacity. Residents were understandably concerned about the congestion that would result from the removal of capacity, and some even foretold of apocalyptic levels of gridlock in the city. However, after the closure of the Embarcadero freeway, while congestion was initially problematic, residents were surprised to find that, after a few months, the city returned to normal. Traffic analysis determined that while many trips simply diverted onto surface streets, a large chunk of the pre-earthquake traffic simply disappeared, as trips were converted to public transport, or grouped together. As a result of this decreased traffic, the city was able to convert the site of the former freeway to a park, improving quality of life and saving millions of dollars.
Further Reading
Wikipedia on Braess’s Paradox
Empirical Evidence on Induced Traffic, Phil B. Goodwin, February 1996 in Transportation Journal
More information on the Embarcadero Freeway