Getting There: From Mobility to Accessibility in Transportation Planning

“An experienced Australian traveler once said that on business trips to Australian cities he could reckon to make four meetings in a day,” wrote John Thomson in 1977. “In Europe he could manage five; in the United States he could manage only three.” The reason behind the variations in this traveler’s itineraries was not an American propensity for long meetings, or the speed of travel in American cities. Instead, his schedules were determined by the great distances—and hence long travel times—separating his business contacts in metropolitan areas of the United States. What the traveler wanted was personal contact with the people with whom he did business. The speed with which he was able to travel was relatively unimportant to him; much more central was the amount of interaction he could accomplish in a given time.

This traveler was unwittingly expressing an account of transportation policy based in accessibility, in contrast to the mobility-centered approach so dominantly reflected in current policy and in the built environment in metropolitan areas around the world. This view extends to the metrics by which transportation systems are assessed. When evaluating the performance of a transportation system, the fundamental criterion for success has long been faster vehicle operating speed. The perspective of the general public is dominated by the idea of mobility-based transportation as well. The widely publicized congestion measures that routinely appear in newspapers nationwide when the Texas Transportation Institute publishes its annual Urban Mobility Report (Schrank and Lomax 2007) have helped to elevate the alleviation of traffic congestion to a top public policy priority. Under all such mobility-based evaluation measures, planners, engineers, and the general public deem rapid movement as a definitive success.

Yet a building block of modern transportation planning is the notion that the demand for transportation is “derived” (Meyer and Miller 1984); that is, people rarely consume transportation for the pleasure of movement per se, but rather travel in order to reach opportunities available at destinations. This fundamental understanding is an underpinning of travel demand analysis, which models transportation flows based on the arrangement of land use patterns across a region. Despite speculation that some market segments may view movement as an end in itself (Salomon and Mokhtarian 1998), the "derived demand" hypothesis remains the consensus of the field, a view supported by the preponderance of empirical evidence.

Apart from its role in travel demand analysis on a land-use basis, the derived-demand assumption has another important implication, which transportation and land-use policy—including investment in roadways, public transit, and walking and cycling facilities, and policy regulating land use—have too rarely confronted. If the purpose of transportation is not movement but access, then increased mobility is desired only to the extent that such a change also increases people’s ability to reach their destinations over the longer run. Pursuit of congestion relief through added transportation capacity can induce destinations to move farther and farther apart (Transportation Research Board 1995). In theory, a paradox can thus arise: increased mobility can be associated, over the long run, with more time and money spent in travel, rather than less. Travel to more remote shopping or work locations might be accomplished at a high speed, but the spread of these destinations can demand more travel than in more compact and clustered urban arrangements in which travel is slower.

If travelers do not consume transportation for its own sake but in order to access destinations, then policies that lead to increased costs per destination would be counterproductive—leaving travelers with less time and money to spend at their destinations. This formulation implies a rejection of mobility or congestion relief as an independent goal for transportation policy. The goal is more properly specified as accessibility, which has been defined as the “potential of opportunities for interaction” (Hansen 1959, 79) or the “ease of reaching places” (Cervero 1996, 1). Mobility, by contrast, is simply the “ease of movement.” Where destinations are nearby, high accessibility can be provided even with low mobility; conversely, where origins and destinations are spread broadly, even great mobility does not ensure high accessibility.

Mobility is one means to accessibility; others are remote connectivity and—notably for urban planning—proximity. Transportation planning’s traditional focus on mobility as its metric of success short-circuits the other two means, and mistakes means for ends. Even the more current version that identifies “accessibility and mobility” as the twin goals of transportation policy is unsupported: it pairs one means with one end, while neglecting the other two means.

The mistaken focus of transportation planning on mobility as the proper goal of policy threatens environmental quality. It resembles the error of gauging the quality of our illumination or computing power through the amount of electricity our lights or computers consume, a move that would suffer from two flaws. First, we would be misjudging the services that we are receiving, which would be more correctly assessed through outcome metrics like lux or million instructions per second, respectively. Second, we would be treating an environmentally consumptive input as an inherently desirable output. “How can we increase the amount of electricity we consume” is the wrong question both in terms of getting what we want (illumination and computing power) and in terms of protecting the environment. We make the same mistake when we ask “how can we increase our mobility” without incorporating the location of our destinations that are the subject of this movement.

The consequences of our use of misguided metrics to guide policy are far-reaching. For example, municipalities regularly decide whether to accept or reject development proposals in part through their impact on roadway level-of-service, a measure of automotive congestion. Most development proposals will increase traffic somewhat, but those that threaten to degrade level-of-service below some minimum threshold trigger requirements either to improve roadways enough to forestall the degradation or to locate elsewhere. The impact of this policy, implemented by thousands of municipalities nationwide, is draconian: development beyond a certain density becomes infeasible, and is therefore pushed to open territory in search of uncongested roadways. In this fashion, the pursuit of mobility ends up degrading metropolitan accessibility.

There is an alternative. Transportation should be evaluated with accessibility metrics to ensure that policy and investment serve transportation’s core purpose, rather than merely facilitating movement. If reforms were implemented, planners could trade off the harm of degraded traffic against the benefits of increased proximity of households to their work and nonwork destinations. Such moves would foster more compact urban spaces because denser development would be welcomed where its proximity benefits outweigh any drag on traffic. By focusing only on the traffic portion of this question, current evaluation approaches actually require a sprawling, auto-oriented urban form by regulation. The focus on market failure—the congestion externality—has blinded transportation and land-use policy to a consequent government failure: development standards that compel low-density development and impede peoples’ ability to choose residences close to their work and nonwork destinations.

The same principle applies to transportation investments themselves. Roadway investments are evaluated through their modeled capacity to ease congestion, with no analysis of their tendency to spur longer trips. Paradoxically, our investments in mobility can themselves degrade accessibility if they induce a spread of origins and destinations that is greater than their impact on congestion reduction. The alternative is to model the impact of transportation on land development, whether of the remote, greenfield variety, or dense infill and redevelopment. This is accomplished today with integrated transportation and land-use models that can support an accessibility analysis of transportation proposals. Those that enhance people’s ability to reach their destinations over the long run are the ones that are worthy of further consideration. Currently the demands of environmental quality are frequently seen as in conflict with people’s need for mobility. Redefining needs in accessibility terms enables pursuit of transportation and environmental goals simultaneously.

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