Induced Demand from Bigger Roads
Traffic has been one of the country’s most aggravating issues, and it always will be. Logically, people think that the solution to heavy traffic is to add more roads and highways. However, this could not be more false. Over the past few decades, traffic engineers have learned that adding more roads and highways does not solve the problems of traffic congestion, but that actually the roads themselves are the problem. The government has made this worse by matching the people’s demand by adding roads for next to nothing, which fuels the issue of induced demand. Induced demand is when “increasing the supply of something (like roads) makes people want that thing even more.”
Two renowned economists, Matthew Turner and Gilles Duranton, discovered a perfect positive relationship between the amount of roads and highways added to particular cities, and the total number of miles driven over those roads during the period. This led them to establish the Fundamental Law of Road Congestion, which states that “new roads will create new drivers, resulting in the intensity of traffic staying the same.” To explain further, when new roads are built, more people will feel a greater inclination and ability to drive to their destinations (i.e. work), increasing the capacity of cars on the road. When the amount of roads and highways are smaller, less people want to drive, with many of them switching to public transit. A great analogy explaining this concept is generally people expect “that expanding a road network works like replacing a small pipe with a bigger one, allowing the water (or cars) to flow better. Instead it’s like the larger pipe is drawing more water into itself.” Of course, there is a limit to Turner and Duranton’s law.
Two famous examples of this phenomenon are the San Francisco removal of the Central Freeway, and the removal of a large highway in Seoul, South Korea. In San Francisco, the freeway previously carried almost 100,000 cars per day, compared to today’s 45,000 daily capacity, but with similar traffic flow. In Seoul, the 168,000 cars per day highway was initially tore down for environmental reasons so they could replace it with a park and river. People were initially greatly concerned about the removal of this highway creating much worse traffic flow. However, the traffic conditions (as well as the environmental conditions) improved.
A solution given by Turner and Duranton is called congestion pricing, where the price of driving would be increased when the road demand is high. This would encourage people who are more flexible to drive at a different time of the day when demand is lower, decreasing the congestion during the high demand times. This has proven to be successful in London, Stockholm, and Singapore. However, when implementation of congestion pricing was attempted in 2008 in New York City, it was voted down because people do not want to pay for something that didn’t cost anything before.
In our INFO 2040 class, we went over this exact phenomenon. In networks terminology, it is called Braess’s Paradox, which states that “adding capacity to a network can sometimes actually slow down traffic.” The example given in the book and in class was one about highways, where the objective was to get from point A to point B (diagram below). One could either pass through C or D to do this. A couple of the edges (which represented the time it would take to drive on those roads) were dependent on the number of cars on the road at that time. However, to exhibit Braess’s Paradox, a superspeedway was added from C to D, drawing all traffic to pass through it. With the added speedway, the Nash Equilibrium of the time it takes to get from A to B increased from before the C-D speedway was there. Highway systems are a prime demonstration of the reality of Braess’s Paradox in networks.
Source: http://www.wired.com/2014/06/wuwt-traffic-induced-demand/