11.4 Sustainable transportation: accessibility, mobility, and

Learning objectives

After reading this module, students should be able to

• explain why the automobile-based system of transportation is unsustainable in terms of inputs, outputs, and social impacts
• explain why transportation is a derived demand and how making transportation sustainable depends on land use as well as vehicles and infrastructure
• differentiate between accessibility and mobility by comparing how they are currently treated by our transportation system
• analyze how a more sustainable system might address accessibility and mobility

What is sustainable transportation?

Transportation is a tricky thing to analyze in the context of sustainability. It consists in part of the built environment: the physical infrastructure of roads, runways, airports, bridges, and rail lines that makes it possible for us to get around. It also consists in part of individual choices: what mode we use to get around (car, bus, bike, plane, etc.), what time of day we travel, how many people we travel with, etc. Finally, it also is made up of institutions: federal and state agencies, oil companies, automobile manufacturers, and transit authorities, all of whom have their own goals and their own ways of shaping the choices we make.

Most importantly, transportation is complicated because it's what is called a derived demand    . With the exception of joyriding or taking a walk or bicycle ride for exercise, very rarely are we traveling just for the sake of moving. We're almost always going from Point A to Point B. What those points are—home, work, school, shopping—and where they're located—downtown, in a shopping mall, near a freeway exit—influence how fast we need to travel, how much we can spend, what mode we're likely to take, etc. The demand for transportation is derived from other, non-transportation activities. So in order to understand transportation sustainability, we have to understand the spatial relationship between where we are, where we want to go, and the infrastructure and vehicles that can help get us there.

Is our current transportation system in the U.S. sustainable? In other words, can we keep doing what we're doing indefinitely? The answer is clearly no, according to professional planners and academics alike. There are three main limitations: energy input, emissions, and social impacts ( Black, 2010 ).

Energy inputs

The first reason that our current transportation system is unsustainable is that the natural resources that power it are finite. The theory of peak oil developed by geologist M. King Hubbert suggests that because the amount of oil in the ground is limited, at some point in time there will be a maximum amount of oil being produced ( Deffeyes, 2002 ). After we reach that peak, there will still be oil to drill, but the cost will gradually rise as it becomes a more and more valuable commodity. The most reliable estimates of the date of peak oil range from 2005 to 2015, meaning that we've probably already passed the point of no return. New technologies do make it possible to increase the amount of oil we can extract, and new reserves, such as the oil shale of Pennsylvania and the Rocky Mountains, can supply us for some years to come (leaving aside the potential for environmental and social damage from fully developing these sites). However, this does not mean we can indefinitely continue to drive gasoline-powered vehicles as much as we currently do.