Induced demand is a popular concept among urbanists, but does its pervasiveness obscure the true costs of mobility?
The January 6, 2023 New York Times story, “Widening Highways Doesn’t Fix Traffic. So Why Do We Keep Doing It?” continues a series of recent media stories that don’t tell the whole story regarding induced roadway traffic. While the theory of induced demand is sound, leaving the impression that induced demand will fully absorb the capacity of new roadway investments or that new demand is somehow bad is very misleading.
Most new demand on expanded roads comes from new population, new employment and economic activity (some or all of which may have been attracted by enhanced transportation infrastructure), traffic rerouted from neighborhood streets or congested roads, or travel that has shifted in time to the benefit of the traveling public now that more capacity is available to undertake activities during desired travel times.
Second, today’s induced demand is likely well below historically estimated levels. Measures of induced demand—and there are many with different definitions and data sources—are based on old empirical data from the last century or very early this century, when travel per person was growing at a few percentage points per year as household incomes, labor force participation, auto availability, and economic activity grew.
Since approximately 2005, U.S. vehicle travel per person has not grown and the amount of travel on roads with newly increased capacity attributable to additional or longer induced trips is likely modest. The National Household Travel Survey documents declining trip rates as telework, e-commerce, and other communications substitution opportunities further dampen travel demand growth pressures, particularly for urban households. If travel expanded to fill all capacity, we would not have any low-volume highways.
More importantly, characterizing induced travel as bad or wasted is a misrepresentation of the value that people derive from engaging in travel. It’s not just wealthy folks making superfluous trips. Residents having access to better jobs or businesses with better selection and lower prices isn’t bad. Businesses having access to a bigger labor pool and potential customer and supplier bases because people can travel farther in a tolerable amount of time isn’t bad. Making supply chains work better isn’t bad. Getting emergency vehicles where they need to go faster isn’t bad. Pulling cut-through traffic out of neighborhoods to travel on a safer highway facility isn’t bad. Having more direct and less congested—and thus environmentally greener—trips isn’t bad. Enabling parents to get home and share a meal with the family isn’t bad. Using transportation infrastructure to shape development or improve economic competitiveness isn’t bad. Being able to engage in social interactions and recreational activities isn’t bad, and contributes positively to physical and mental health.
There is a lot of literature on induced vehicle traffic, but beyond theory and research, additional issues now merit attention from policymakers and transportation professionals.
How important is mobility—and whose mobility is important?
Multiple congressional legislative cycles, successive executive administrations, all states, and hundreds of local and regional transportation agencies have mission statements and goals for their transportation programs that aspire to improve mobility for people and goods. Mobility is widely recognized as critical to economic competitiveness, productivity, upward mobility, and quality of life. Recognizing the importance of mobility, cities are now making extraordinary efforts to ensure mobility for all segments of the population. Public transportation planners are also paying more attention to mobility for private sector freight, recognizing its importance to transportation, supply chains, and the economy. Roadways accommodate the vast majority of surface freight transportation and virtually 100 percent of last-mile freight and product transportation. Roadways accommodate about 99 percent of person vehicle miles of surface travel plus about one-half of public transit trips. Even for individuals who seldom use the roadway network, the construction of their residence, the items in it, the utilities and persons that serve it, and the waste generated by their household are transported via the road system.
Congestion is an onerous time tax that does not discriminate. It undermines mobility for rich and poor, able-bodied and impaired, and travelers of all races and ethnicities. It impacts people, businesses, and public services. The impacts of failing to provide needed capacity are not restricted to affluent folks making unnecessary trips; in fact, low-income workers can be severely impacted by congestion delays and travel time unreliability.
Overplaying induced demand arguments as a pretext for discouraging roadway expansion or presuming travel demand can be accommodated by investing in alternative modes can be disingenuous and ill-informed. The presumption that directing resources to transit, bike, or pedestrian options can meet the mobility needs of all people and goods seldom works out as a real solution that is financially sustainable, environmentally superior, or offers comparable mobility, accessibility, or other benefits.
Paying attention to population and travel growth when evaluating capacity needs
When people migrate between geographies in the United States, they don’t bring their transportation infrastructure with them. Therefore, absent excess capacity in the new location or unprecedented changes in travel behavior, they will create a need for new capacity in their new locations. For example, about 65,000 New York residents traded their New York driver’s licenses for Florida licenses in 2022. Based on the national average of lane miles per capita, they should each bring about 133 linear feet of roadway lane with them to accommodate their travel in Florida. That totals about 1,638 lane miles, plus more if you count the children and persons not getting Florida licenses. Adding all the net state and international migration over multiple years makes it obvious that new roadway infrastructure is required to meet mobility needs in growing areas.
On the other hand, travel demand per capita has plateaued or is shrinking, and many areas are not growing in population. Between 2010 and 2020, 1,010 of 3,148 counties in the United States lost population. Between 2020 and 2021, 1,318, or 41 percent lost population. Ten states lost population between 2015 and 2020. Ten of the largest 50 incorporated areas lost population between 2016 and 2019.
Roadway investments in new capacity should be based on up-to-date and sound demand estimates. They can't just fulfill out-of-date plans or serve as ill-advised opportunities to create jobs or garner state and federal resources for local use. They should not use twentieth-century per capita travel growth rates or chamber of commerce-inspired population growth assumptions. They should not assume telework or communications substitution for travel will revert to pre-COVID levels. They should be based on a robust analysis of options and alternative modes for meeting mobility needs. Objective analysis should be a prerequisite for planning for any mode. Some communities need to face their demographic reality and should exercise a high degree of discipline in building new capacity if they're failing to maintain existing infrastructure.
Does public transit induce roadway traffic?
The premise that new roadway capacity induces traffic, if applied consistently within transportation systems analysis, means that any initiative that attracts travelers to an alternative mode or otherwise reduces roadway travel simultaneously induces replacement travel due to the now available roadway capacity. For example, the analysis of a new transit project would need to debit the energy, emissions, noise, safety, and other benefits attributed to transit by the impacts of the induced roadway traffic. An objective analysis of transportation requires consistent treatment of induced travel across modes and projects. This requires a rich understanding of induced demand, including the impact of context-specific variables and how they may influence induced demand for a specific set of demographic, economic, geographic, and other conditions.
Planning can be sensitive to induced demand
Much of the reporting on induced demand gives the impression that the transportation planning community is oblivious to this phenomenon or is comprised of road-building zealots. Newer activity-based transportation models are designed such that activity generation (trip generation) is sensitive to travel times. Consequently, improvements in travel speed will contribute to predictions of increased trip-making and travel distance. Even without the newest models, scenario testing and careful analysis of changes in demographics, mode choices, and flow volumes and patterns can give insight into the nature of demand on new facilities.
Planners must also consider that new roadway infrastructure planned or built today is destined to spend most of its useful life supporting the conveyance of electrically propelled and sustainably powered vehicles. Perhaps more importantly, roadway infrastructure built now can be configured and equipped with technologies to support the operation of freight vehicles, priced lanes, public transit services, automated vehicle lanes, or various combinations of vehicles to enable the productive use of the roadway.
As planners explore the need for new capacity, they need to value mobility just as they try to understand and mitigate its impacts. Mobility should not come at any cost, but neither should its value be discounted.
Select media pieces on induced demand
“Widening Highways Doesn’t Fix Traffic. So Why Do We Keep Doing It?,” The New York Times, January 6, 2023
“Calculating ‘induced demand’,” PUBLIC SQUARE, A CNU Journal, Robert Steuteville, Dec. 13, 2021
“Induced Demand Is Hard to Explain – But It’s Crucial to Get It,” STREETBLOG, Melanie Curry, Mar 2, 2022
“Why the Concept of Induced Demand Is a Hard Sell,” Governing, Jake Blumgart, Feb. 28, 2022
“More highways, more driving, more emissions: Explaining ‘induced demand’,” Smart Growth America, Steve Davis, October 20, 2021
“Why Don't State DOTs Believe in Induced Demand?” Bloomberg CityLab, Oct 6, 2021
“Spreading the Gospel of Induced Demand,” Transfersmagazine, Issue 9, Nicholas J. Klein et. al., June 2022
“How highways make traffic worse,” Vox, Laura Bult Feb 12, 2021
“Will Parson’s proposed expansion of I-70 reduce congestion? Here’s what research shows,” The Kansas City Star, Johnathan Shorman and Kacen Bayless, January 20, 2023
“The Unstoppable Appeal of Highway Expansion,” Bloomberg CityLab, David Zipper, September 28, 2021
For more thoughts on induced traffic and links to other references see Induced Demand’s Effect on Freeway Expansion.
Dr. Steven Polzin is a Research Professor at Arizona State University School of Sustainable Engineering and the Built Environment. Prior engagements include serving as a Senior Advisor for Research and Technology at USDOT, Director of Mobility Policy Research at the Center for Urban Transportation Research, University of South Florida, and working for transit agencies in Dallas, Cleveland, and Chicago.
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