Transportation networks enable movement of people and products, which is essential to a well-functioning society. Designing effective and environmentally responsible networks is challenging, but not impossible—careful planning and an understanding of how humans interact in complex networks are essential.
Human problems, human solutions
The physical movement of humans and products over space and time is essential to the functioning of societies and economies. Transportation networks make this possible and directly affect everyone as we travel to work, to school, to visit friends and family, to shop, and to attend sporting and cultural events.
These fundamental movements that sustain and grow societies cannot be realized without vehicles, the production of which makes its own economic contribution, comprising one of the largest manufacturing industries in the United States. Ground transportation alone contributed in 1994 $900 billion to U.S. gross domestic product (GDP). In the middle of the 20th century, there were 2.6 billion people on Earth and 50 million cars (Nagurney 2000). Roughly three generations later, the number of people has risen to 7 billion and the total number of cars to over 600 million, a more than twelve-fold increase in cars while the population had only increased by a factor of less than 3.
Nevertheless, despite the economic gains, population mobility, and other positives of vehicle ownership and use, the negative effects—notably, traffic congestion, greenhouse gas emissions, and accidents—are now well-recognized. For example, in the United States, congestion results in over $100 billion in lost productivity annually (Texas Transportation Institute 2011). The impacts of traffic congestion, including wasted time, frustration, and losses in productivity, are not insignificant and its effect on energy consumption is immense.
In the past two decades, drivers in the United States have experienced an increase in wasted fuel of 440 percent due to congestion, even though the energy intensity of automobiles has decreased by 20 percent. According to the Texas Transportation Institute, a typical traveler in the Washington, DC metropolitan area wasted 74 hours stuck in traffic in 2010, about the same amount of time as she had for vacation. Congestion and its impact on energy consumption, however, is not only a passenger vehicular problem. It also affects the movement of products and their prices in today’s increasingly globalized supply chains.
Perhaps one of the most pressing public policy problems related to congestion is environmental degradation. The negative effects of air pollution from vehicular exhaust emissions, for example, are well-documented, with a typical passenger vehicle or car emitting 1.49 metric tons of carbon annually (U.S. Environmental Protection Agency 2005). Congestion and idling are critically linked to increased emissions.
Despite 25 years of engineering progress and substantial reductions in transportation-related emissions, motor vehicles remain important sources of emissions of carbon monoxide, organic compounds, nitrogen oxides, and other forms of air pollution, mainly due to the growth of vehicular fleets. Presently, about 15 percent of the world’s emissions of carbon dioxide, the principal greenhouse gas responsible for global warming, is generated by motor vehicles. This can be daunting, but we must realize that these are problems caused by human decisions; with careful analysis and smart policy design, these problems are far from insurmountable.
Policies and habits
We can note numbers and talk about “vehicle congestion,” but it’s important to keep in mind that humans are ultimately those who do the driving, and select modes of transportation—from walking and bicycling to using private cars or mass transit. Humans also make the purchasing decisions and choose where to live, work, got to school, and how and when to conduct their activities. We are not the first generation to be worried about transportation and congestion, nor will we be the last. The Romans were concerned about congestion in central Rome during ancient times and instituted a time-of-day chariot policy whereby individual chariots were banned from central Rome during certain hours of the day. A similar policy was in effect in New York City in December 2005 during the short-lived transit strike whereby only cars with four occupants or more could enter parts of Manhattan.
Congestion pricing is another transportation policy instrument that can mitigate the negative externalities associated with congestion and the accompanying pollution. Congestion pricing has been recognized since the early 1900s and has been implemented in various parts of the globe, showing increasing promise due to advances in technology, including electronic tolls. Under such a system, surcharges exist for the use of selected congested routes during peak traffic periods, shifting some vehicular trips to off-peak periods or to routes away from the congested ones. Some travelers may select different modes of transportation or higher-occupancy vehicles, and some may choose not to travel altogether, opting instead to telecommute. London’s experiment with congestion pricing has been deemed a success and Oslo, Singapore, and Stockholm charge similar tolls. Revenues from such policies can be used to provide improved transportation alternatives. Such pricing systems are being implemented in various parts of the globe as means to acquire funds for further road construction.
Congestion policies can help reduce emissions and keep societies moving, and so too can changes in human behavior—from such simple actions as checking tire pressure, obeying speed limits, keeping cars tuned, and using fuel-efficient engine oils are all eminently doable and have an impact. Taking action to reduce congestion and energy consumption by car-pooling, using mass transit, and bicycling and walking whenever feasible can also make major positive impacts on air quality. Indeed, public transportation in particular can be very effective at achieving congestion- and emissions-reduction goals when it is efficient, comfortable, reliable, and competitively priced.
Better Design of Transportation Networks
Beyond personal decisions and policies that encourage better driving habits, there exist opportunities on a grander scale—in the design of transportation networks. In 1968, Dietrich Braess demonstrated that the addition of a new road (without a change in travel demand) could, in fact, make everyone worse off in terms of travel time. This has since become known as the Braess paradox and instances have been identified—in New York City, for example, when 42nd Street was closed on Earth Day in 1990 and travel time actually improved. More recent vivid examples have included the removal of the highway in downtown Seoul, Korea, and the resurrection of the original riverway as well as the closure of Broadway in New York City from 47th to 42nd Street since May 2009.
The original 1968 Braess article was in written German, and was translated in 2005 by my colleagues and me, appearing then in Transportation Science along with a preface (Nagurney and Boyce 2005), which highlighted how Braess rediscovered the two fundamental principles of travel behavior or transportation network operation, in addition to the original paradox named after him. My earlier research has shown how to add roads so that the Braess paradox never occurs. As a fascinating aside, the occurrence of the Braess paradox on the Internet has now been recognized. That network is increasingly characterized by behaviors of users similar to those of travelers on transportation networks, who act independently and in a decentralized manner in choosing their optimal routes of travel between origins and their destinations.
And yet, crafting policy with the Braess paradox in mind is not the only solution—challenges exist beyond understanding and incorporating Braess’s insights. My colleagues and I have also shown that so-called “improvements” to a transportation network may actually result in an increase in total emissions. What’s more, that the addition of a link that generates zero emissions (a telecommuting policy, for example) may result in an overall increase in emissions. The results depend crucially on the behavior of the individuals concerned, the structure of the underlying networks, and the associated costs. And thus the addition or deletion of new modes to existing transportation networks need to be carefully assessed before changes are implemented, and considerations for the behavior of travelers must be included in such assessments.
In summary, the design of networks for prosperity and sustainability should include not only meet standards of scientific and engineering rigor, but must be coupled with a full understanding of the critical human component.
Braess, D., A. Nagurney, and T. Wakolbinger. 2005. “On a Paradox of Traffic Planning,” translated from the original (1968) Braess paper from German to English. Transportation Science 39: 446–450.
U.S. Environmental Protection Agency. 2005. Overview: Pollutants and Programs. http://www.epa.gov/otaq/climate/420f05004.htm
Nagurney, A. 2000. Sustainable Transportation Networks. Cheltenham, England: Edward Elgar Publishing.
Nagurney, A., and D. Boyce. 2005. “Preface to ‘On a Paradox of Traffic Planning.’” Transportation Science 39: 443–445.
Texas Transportation Institute. 2011. Urban Mobility Report. http://mobility.tamu.edu/ums