Elements of Access: Induced Demand

Induced Demand
Induced Demand


by Wes Marshall

You already have a congested roadway, and the transportation planners predict even more traffic on that road in the near future. What do you do? For most of the last century, the answer was to increase capacity. In the short-term, this seemed to work. Time and time again, over the long-term, the actual amount of traffic after the capacity increase grew far more than expected. What seemed like an obvious solution to a congestion problem continued to disappoint. But why?

The reason for these failures lies with the principle of induced demand. Once capacity increases, not only do you get the originally predicted traffic growth, but you also facilitate some often unanticipated changes in travel behavior. First, existing road users might change the time of day when they travel; instead of leaving at 5 AM to beat traffic, the newly widened road entices them to leave for work with everyone else. Second, those traveling a different route might switch and drive along the newly widened option. Third, those previously using other modes such as transit, walking, bicycling, or even carpooling may now decide to drive or drive alone instead. Together, these unwanted behavior changes fall under what is termed the theory of triple convergence (also known as the ‘Iron Law of Congestion’). This latent demand induces more traffic than originally expected and saps the supposed improvement of the expected benefits.

The joke is that adding lanes to cure congestion is like loosening your belt to cure obesity. Empirical results over the last century – due to the principle of induced demand – have borne out that this issue is real and should always be accounted for when considering adding capacity as a solution to congestion.







Downs, A. (2004). Still Stuck in Traffic. Washington DC, Brookings Institute.



William Louis Garrison (1924-2015)

Saddened to learn of the passing of my co-author and teacher, Bill Garrison.

Article from UC Berkeley Civil and Environmental Engineering News

William Louis Garrison, UC Berkeley professor emeritus in the Department of Civil and Environmental Engineering and former director of Berkeley’s Institute of Transportation Studies, died on Feb. 1, 2015 in Lafayette, CA. He was 90.

Garrison joined the CEE faculty as a Professor in 1973. That same year he was appointed director of the Institute of Transportation and Traffic Engineering (ITTE) (the predecessor of the Insitute of Transportation Studies), a position he held for 7 years. He retired from Berkeley in 1991.

Although his work at Berkeley was focused on how innovation and technological change occurs in the field of transportation, he was well known earlier in his career for leading the so-called “quantitative revolution” in geography.

“Bill moved from geography to transportation in the 1970s. He was particularly interested in innovation and how technological change occurs,” said David Levinson, a former student of Garrison’s who now serves on the faculty of the Department of Civil, Environmental and Geo-Engineering at the University of Minnesota.

“He strongly believed that we should be seeking new development pathways, rather than remaining stuck perfecting existing systems—what he called the ‘polished present’.”

Another former student, Barry Wellar commented in 2007 at the 2007 Anderson Distinguished Lecture in Applied Geography that Garrison “had a way of looking at things that were way outside the box years before that notion was popularized, and even his so-called ‘easy reads’ contained nuances about relationships and processes that would be missed by the casual reader.”

Garrison was born in 1924 and raised in Tennessee. Following his service in World War II during which he did meteorological work for the US Army, he received his PhD in Geography from Northwestern University in 1950.

A few years later, as a young faculty member at the University of Washington, he led the way to revitalizing the field of geography through the use of greater scientific thinking and methods. That led to an increased use of computerized statistical techniques such as multivariate analysis in geographical research. Garrison and his students used such historic computing systems as the IBM 604 and IBM 650. Many of those students would later go on to revolutionize geographic science and geographic information systems.

In recent years, the biennial William L. Garrison Award for Best Dissertation in Computational Geography was created to recognize Garrison’s outstanding research and educational contributions. The award is intended to arouse a more general and deeper understanding of the important role that advanced computation can play in resolving the complex problems of space–time analysis that are at the core of geographic science.

In addition to the University of Washington, Garrison taught at Northwestern University, University of Pennsylvania, University of Illinois, University of Pittsburgh, before he arrived at UC Berkeley in 1973 as Professor in the Civil Engineering Department, and ITTE Director.

By then his interests had shifted to transportation. He made invaluable contributions to the Transportation Engineering Program in the department, expanding and strengthening the planning and policy elements of the curriculum. Likewise, when he took at the helm at ITTE he set out to expand and broaden the scope beyond transportation and traffic engineering.

“Bill steered the insitute into a broad based center for the study of transportation. He expanded its agenda and broadened the community of faculty affiliates to encompass many departments on the campus including City and Regional Planning, Economics, Geography, Public Policy and Sociology,” said Adib Kanafani, CEE Professor of the Graduate School. Kanafani succeeded Garrison as ITS Director in 1983.

“Bill’s leadership was reflected in the institute’s name change as ITTE became the “Institute of Transportation Studies.”

Garrison served on numerous national committees, and served as Chair of the Executive Committee  of the Transportation Research Board in 1973. In the late 1980s he spent time in Austria examining growth trajectories of various transportation technologies. Much of that work is what forms the nucleus of The Transportation Experience first published by Oxford University Press in 2006, then revised and published in a second edition in 2014. His co-author was David Levinson.

“He was one of the few people to take a macro-view of transportation and who tried to understand the long-term dynamics of systems,” said Levinson.

In 2007, Garrison gave the Anderson Distinguished lecture in Applied Geography at the Association of American Geographers in Washington, DC.

In his address, he said:

“About fifty years ago I began to teach a course on transportation geography, and at about that same time I began working with others on highway improvement and financing in Washington State and nationwide telephone communications as a civil defense matter. In subsequent decades I have worked to understand transportation systems of many types as well as engineered systems such as sanitary systems. This work raised many questions of a ‘why do systems and actors do what they do’ sort. To comment on those questions I will refer to constraints formed by networks and institutional structures.”

Systems are birthed when actors combine old or new building blocks and produce services aligned with markets. Early on, there is flexibility as the marriage of systems and markets is forged.

The early decades of the railroads provide a sweeping example. Created by mixing and matching building blocks from tramways, steam engines, canal and toll road construction techniques and pricing protocols and management and financing techniques from military, church and industry they molded themselves like clay. Discovery was a key theme—discovering a workable mix of hard and soft technologies and, equally important, discovering markets. Discovery extended to finding improvements in technologies to respond to social innovations, such as those asking for expanded passenger transportation. Railroads adjusted themselves to circumstances of market density, raw materials for fuel and construction, and styles of national governance. They were flexible.

The flexible period ran from about 1830 to 1860. Afterwards, deployment and growth ruled, and network and organizational inflexibilities began to exert themselves, along with those imposed by those striving to control development using standards, regulations, and other tools. Flexibility became more and more restrained…

I recognize that our legacy infrastructure systems have enormous value. They serve as inputs for all that we do, and shape social and economic structures. They form a record of learning and social and economic actions and achievements. But at the same time, their inflexibilities are costly now and place serious limits on options for the future.”

William Louis Garrison is survived by his wife Marcia Garrison and their four children, Deborah Churich, James Garrison, Jane Garrison Grimaldi, and John Garrison; his three children, Sara Garrison, Ann Darrin, and Helen Saxenian from his first wife Mary Margaret Garrison (who predeceased him); 16 grandchildren, and one great grandchild.

Memorial services will be private at the request of the family.

Minnesotans driving less, biking and walking more A study shows Minnesotans were traveling less in 2010 than in 2000. | MnDaily

Benjamin Farniok writes in the Minnesota Daily “Minnesotans driving less, biking and walking more A study shows Minnesotans were traveling less in 2010 than in 2000.”

Minnesotans aren’t traveling as much as in past years, according to a study by University of Minnesota researchers released earlier this month.

The findings may inform decisions regarding future transportation infrastructure and policy.

Overall, Minnesotans aren’t traveling as often because of demographic and economic changes, said David Levinson, a civil engineering professor and the project’s primary investigator.

The number of trips people took each day, including biking and walking, decreased from 11.6 million in 2000 to 9.8 million in 2010.

Levinson also said that more 16- to 18-year-olds are holding off getting their driver’s license, and in total there are fewer licensed drivers per household today than in 1970.

Bicycle trips also increased 13 percent between 2000 and 2010, according to the survey, although women do not bike as much as men.

“A common argument against this kind of research is, ‘Why does it matter that women are not biking as much as men?’ … We can see, from the fact that there is a difference, that our transportation system isn’t serving people’s needs in the same way,” said Jessica Schoner, a graduate student who worked on the project.

There were a number of issues in comparing the surveys, Schoner said. For example, the 2010 study surveyed more people and also asked different questions, she said, which could have changed how the data is presented.

This is about the Travel Behavior Over Time study, which is coming out “later this year”.

Elements of Access: Layers

Layers and Layers

  1. Site: Right of Way & Alignment (Horizontal and Vertical)
  2. Street: Structure (Bridges, Pavement, Tracks, Runways etc.)
  3. Signs and markings
  4. Signals
  5. Vehicle
  6. Driver
  7. Purpose of travel (Service)

Transportation systems are designed and constructed as interdependent layered networks. The physical systems are layered as shown in the Figure, in an analogy with the Open Systems Interconnection (OSI) model of communication layers that define the internet. The hierarchy of services include short- and long-distance transportation services (e.g., buses, freight) that make use of different and overlapping parts of the street network.

The question of resilience is relevant to most of these layers. Transportation will be interrupted to a greater or lesser extent if the physical alignment is blocked (e.g., flood, snow, hazmat leak), if the road or bridge structure fails (e.g., potholes, sinkholes, heaving, a bridge collapse), if traffic control devices (signs, markings, signals) fail or deteriorate (e.g., an electrical outage, weathering), if vehicles fail to work (e.g., gasoline shortages, electrical outages in the case of electric vehicles, or system-wide vehicle recalls), if the driver is unable to drive the vehicle (e.g., strikes or illness), or if a service is interrupted (e.g., airplanes are grounded, trains or buses are canceled, traveler information systems are disrupted). Overuse of transportation facilities (congestion) may lead to a type of traffic failure that results when travel times increase beyond an acceptable level (or the area reachable in a given time shrinks). This type of failure can have consequences ranging from annoying to critical.

Within these layered networks are a hierarchy of roads and services. The hierarchy of roads is an emergent phenomenon that has become enmeshed in road and highway design – some roads are more significant (faster, more intensively used) than others. Similarly, transit systems are often designed with feeder routes connecting with regional routes, and freight systems have collector / distributor networks (local delivery networks) that connect to a long distance system. This hierarchical arrangement is also common in other networked utility systems (electrical, natural gas, water, etc.).


  • Levinson, D. and K. Krizek (2008). Planning for Place and Plexus: Metropolitan Land Use and Transit. New York City: Routledge.
  • Stallings, W. (1987). Handbook of computer-communications standards; Vol. 1: the open systems interconnection (OSI) model and OSI-related standards. Macmillan Publishing Co., Inc.
  • Townsend, A. (2012). Re-programming mobility: How the tech industry is driving us towards a crisis in transportation planning. Technical report, New Cities Foundation.
  • Levinson, D. and B. Yerra (2006). Self organization of surface transportation networks. Transportation Science 40(2), 179–188.
  •  Yerra, B. M. and D. M. Levinson (2005). The emergence of hierarchy in transportation networks. The Annals of Regional Science 39(3), 541–553.

Our hatred of taxes, demands for services conflict | Wilmington News Journal

Wilmington News Journal writes an Opinion: Our hatred of taxes, demands for services conflict.

As Delaware Transportation Secretary Jennifer Cohan notes, no community leader will deny the state has a funding problem for its roadways. However, as those same community leaders will acknowledge, there is practically no way we are going to solve that problem. Americans in general, and Delawareans in particular, do not like paying taxes. They never did. In recent years the hostility to taxes are grown. The anti-tax movement remains strong and any proposed tax on gasoline makes it even stronger.

The result is, as Secretary Cohan told legislative budget writers last week, $600 million worth of highway projects will be on hold for years. That sounds reasonable, until the missing roadwork affects them. Then it gets personal. The demand for services will go up. However, there still will not be tax increases.

David Levinson, a professor of transportation studies at the University of Minnesota, explains it this way: “Roads are governed by elected officials, who believe they are re-elected when they keep taxes down and are sometimes punished when they raise taxes.” Voters do not trust transportation departments, Professor Levinson says, and they are not always wrong.

Congress is in the same shape as the Delaware General Assembly. It is supposed to fix the federal highway trust fund. However, it seems highly reluctant to do so if it adds anything to the taxpayers’ gasoline bill. Economists believe a way can be found so that the private cost of driving a car comes close to the actual public cost. Despite what we pay at the pump or for tolls or fees, the cost of using the roads and polluting the air is much higher. Economists, both right and left, argue this should be evened out.

Economists, however, do not run for re-election.

The largest source of revenue for the road projects are the tolls on I-95 and Del. 1. This makes sense and follows the same no-tax logic of gasoline levies. The Del. 1 toll is higher on the weekend. The drivers paying that extra money are usually driving for pleasure, not work. The extra cash goes down more easily.

The I-95 tolls is better yet from this point of view. It is a border tax, if you will. The bulk of it is paid by people driving through Delaware. They are highly unlikely to protest a high toll by voting against Delaware incumbents. Our trouble is that we do not have more borders that strangers want to cross.

Every driver has welcomed the current lower price of gasoline. Tax increase suggestions from infrastructure advocates have been shouted down, both by the diehard anti-tax groups and the average motorist who is enjoying the relief. Therefore, the impasse continues. Cars will become even more efficient on gasoline and thus lower the amount of tax coming in. Postponed maintenance will grow more costly and wear and tear on the roads will get worse.

Somewhere along the line, something will have to give. Even re-election wary politicians will be forced to agree.

Delaware of course has some of the highest share of revenue from tolls in the US due to its strategic position of owning a short turnpike between Maryland, Pennsylvania, and New Jersey, of being able to tax foreigners living abroad. Yet even they face the same issue as other states as to how to raise revenue.

Speculative Hypothesis: Planners’ discount rates are too low

When we are children, we learn the fable of the ant and the grasshopper. Briefly, the ant saves food for winter, the grasshopper doesn’t, and instead goes clubbing. The ant survives, the grasshopper doesn’t. Moral lesson: plan for the future, food isn’t as plentiful in the winter.

Dancing Grasshopper. Source: http://srujanakktrip.blogspot.com/2011/09/ant-and-grasshopper.html
Dancing Grasshopper. Source:

And certainly we should all plan to some extent. Today the personal economic issue is retirement, and many people don’t save enough in the US to live as well as they might like upon ceasing full-time work. This is in part because they lived too well while they were working instead of deferring pleasure.

But there is the opposite view, which is that of “live for today, for tomorrow we may all be dead”. Saving for the future does you no good if you have no future. The earth might be destroyed by an asteroid (think about the poor dinosaurs who spent their lives saving and instead of clubbing, where did that get them?). Or we might destroy ourselves (when I was a child, the boogey-man was Nuclear War, then Nuclear Winter, then Acid Rain, then the Ozone Hole, now Global Warming … there is always a boogey-man). Or our society might be undermined by an outside threat: Emmanuel Goldstein, Osama bin Laden, Abu Abdullah al-Rashid al-Baghdadi.

So how much to save depends on the likelihood of survival and persistence. If I expect to live, or at least be able to transfer my wealth to descendants, my motivation for savings is higher. If I expect to die, or have my assets seized, or depreciated in a major round of hyperinflation, really what is the point?

People engaged in the planning field have by their very nature a longer time-horizon than random. They are looking at public works that will take decades to get built (and years to build), and last many more decades if not centuries.

Forecasts predicting gloom and doom (the scare forecast) (too much congestion on a map painted red) far into the future are “bringing distant dangers near”. We have no reason to plan for tomorrow’s possible problems when we have plenty of problems staring us in the face

From a financial economic perspective, it is often unwise to make these long-term speculative investments, or even plan to make these investments, their payback period is too long, the technology may change in the interim. Resources could be better allocated to things which can be made, and innovated, in a much shorter time horizon. The modern version of capitalism ruthlessly (eventually) punishes these investments.

Though I don’t think a formal study has been done, much of the private capital invested in transportation infrastructure historically has been  wiped out [there are many reasons for this, not all of which are market related, some due to regulation], which is why such investments have most recently been mostly left to the public sector. (And recent examples of private infrastructure investment have been nothing to write home about: Dulles Greenway, Indiana Toll Road, the Channel Tunnel, the London Underground are just a few of the high profile disasters.) At a minimum, getting this right is hard.

The discount rate is how we discount future money back to the present, it  the interest rate you earn on investments looked at from the other direction. Are you indifferent to $1.00 today or $1.10 a year from now. Then your discount rate is 10%. The planner’s discount rate asymptotically approaches 0%.

Robin Hanson at Overcoming Bias makes an interesting distinction between “far” and “near” modes of thinking. We are more idealistic in “far” mode. While almost everyone operates in both modes some of the time, some people operate in one mode more often. Hanson points out that “Disagreement is Far“, himself quoting:

Recruiting a sample of Americans via the internet, they polled participants on a set of contentious US policy issues, such as imposing sanctions on Iran, healthcare and approaches to carbon emissions. One group was asked to give their opinion and then provide reasons for why they held that view. This group got the opportunity to put their side of the issue, in the same way anyone in an argument or debate has a chance to argue their case.

Those in the second group did something subtly different. Rather that provide reasons, they were asked to explain how the policy they were advocating would work. They were asked to trace, step by step, from start to finish, the causal path from the policy to the effects it was supposed to have.

The results were clear. People who provided reasons remained as convinced of their positions as they had been before the experiment. Those who were asked to provide explanations softened their views, and reported a correspondingly larger drop in how they rated their understanding of the issues. (morepaper; HT Elliot Olds)


Long Range Transportation Planners in practice seldom lay out actual deployment paths, and instead focus on the end state, a vision, a 30-year plan.  A different set of people (or sometimes the same people in a different set of roles) put together a 6-year Capital Improvement Plan. A third set of people deals with annual budgets. A fourth set operationalizes that. A fifth set actual builds and delivers the projects.

As the wise show Parks and Recreation said “There are no planning emergencies”. With the attitude in contrast to most people who live in the now and with urgency, who spend their lives metaphorically if not actually “fighting fires”, it is no wonder that planners cannot meet deadlines. In their worldview, there are no deadlines.

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