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.

Scientists are working on the development of alternative fuels such as biofuels or hydrogen, but these have their own limitations. For example, a significant amount of land area is required to produce crops for biofuels; if we converted every single acre of corn grown in the U.S. to ethanol, it would provide 10% of our transportation energy needs. Furthermore, growing crops for fuel rather than food has already sparked price increases and protests in less-developed countries around the world (IMF, 2010). Is it fair to ask someone living on less then two dollars a day to pay half again as much for their food so we can drive wherever and whenever we want?

The engine of the typical automobile or truck emits all sorts of noxious outputs. Some of them, including sulfur dioxides, carbon monoxide, and particulate matter, are directly harmful to humans; they irritate our lungs and make it hard for us to breathe. (Plants are damaged in much the same way). These emissions come from either impure fuel or incomplete burning of fuel within an engine. Other noxious outputs cause harm indirectly. Nitrous oxides (the stuff that makes smog look brown) from exhaust, for example, interact with oxygen in the presence of sunlight (which is why smog is worse in Los Angeles and Houston), and ozone also damages our lungs.

Carbon dioxide, another emission that causes harm indirectly, is the most prevalent greenhouse gas (GHG), and transportation accounts for 23% of the CO₂ generated in the U.S. This is more than residential, commercial, or industrial users, behind only electrical power generation (DOE, 2009). Of course, as was explained above, transportation is a derived demand, so to say that transportation itself is generating carbon emissions is somewhat misleading. The distance between activities, the modes we choose to get between them, and the amount of stuff we consume and where it is manufactured, all contribute to that derived demand and must be addressed in order to reduce GHG emissions from transportation.

If the definition of sustainability includes meeting the needs of the present population as well as the future, our current transportation system is a failure. Within most of the U.S., lack of access to a personal automobile means greatly reduced travel or none at all. For people who are too young, too old, or physically unable to drive, this means asking others for rides, relying heavily on under-funded public transit systems, or simply not traveling. Consider, for example, how children in the U.S. travel to and from school. In 1970, about 50% of school-aged children walked or biked to school, but by 2001, that number had dropped to 15% (Appleyard, 2005). At the same time that childhood obesity and diabetes are rising, children are getting less and less exercise, even something as simple as walking to school. Furthermore, parents dropping off their children at school can increase traffic levels by 20 to 25%, not just at the school itself, but also throughout the town in question (Appleyard, 2005). At the other end of the age spectrum, elderly people may be functionally trapped in their homes if they are unable to drive and lack another means of getting to shopping, health care, social activities, etc. Finally, Hurricane Katrina made it clear that access to a car can actually be a matter of life or death: the evacuation of New Orleans worked very well for people with cars, but hundreds died because they didn't have the ability to drive away.

Another serious social impact of our transportation system is traffic accidents. Road accidents and fatalities are accepted as a part of life, even though 42,000 people die every year on the road in the U.S. This means that cars are responsible for more deaths than either guns, drugs, or alcohol (Xu et al., 2010). On the bright side, there has been a steady reduction in road fatalities over the last few decades, thanks to a combination of more safety features in vehicles and stricter enforcement and penalties for drunk or distracted drivers. Nevertheless, in many other countries around the world, traffic accidents are in the top ten or even top five causes of death, leading the World Health Organization to consider traffic accidents a public health problem.

An additional problem with our current unsustainable transportation system is that much of the rest of the world is trying to emulate it. The U.S. market for cars is saturated, meaning that basically everyone who can afford or is likely to own a car already has one. This is why automobile manufacturers vie so fiercely with their advertising, because they know they are competing with each other for pieces of a pie that's not getting any bigger. In other countries such as China and India, though, there are literally billions of people who do not own cars. Now that smaller, cheaper vehicles like the Tata are entering these markets, rates of car ownership are rising dramatically. While the same problems with resources, emissions, and social impacts are starting to occur in the developing world, there are also unique problems. These include a lack of infrastructure, which leads to monumental traffic jams; a need for sharing the road with pedestrians and animals; and insufficient regulation to keep lead and other harmful additives out of gasoline and thus the air.

This is the hardest category to rely on for a solution, because we simply can't predict what might be invented in the next five to fifty years that could transform how we travel. The jet engine totally changed air travel, making larger planes possible and increasing the distance those planes could reach without refueling, leading to the replacement of train and ship travel over long distances. However, the jet engine has not really changed since the 1960s. Is there some new technology that could provide more propulsion with fewer inputs and emissions? It's possible. But at the same time, it would be unreasonable to count on future inventions magically removing our sustainability problems rather than working with what we already have.

Technology is more than just machines and computers, of course; it also depends on how people use it. When the automobile was first invented, it was seen as a vehicle for leisure trips into the country, not a way to get around every day. As people reshaped the landscape to accommodate cars with wider, paved roads and large parking lots, more people made use of the car to go to work or shopping, and it became integrated into daily life. The unintended consequences of technology are therefore another reason to be wary about relying on new technology to sustain our current system.

The economist Anthony Downs has written that traffic jams during rush hour are a good thing, because they indicate that infrastructure is useful and a lot of people are using it (Downs, 1992). He also notes that building more lanes on a highway is not a solution to congestion, because people who were staying away from the road during rush hour (by traveling at different times, along different routes, or by a different mode) will now start to use the wider road, and it will become just as congested as it was before it was widened. His point is that the road itself is a resource, and when people are using it for free, they will overuse it. If instead, variable tolls were charged depending on how crowded the road was—in other words, how much empty pavement is available—people would choose to either pay the toll (which could then be invested in alternative routes or modes) or stay off the road during congested times. The point is that every car on the road is taking up space that they aren't paying for and therefore slowing down the other people around them; charging a small amount for that space is one way of recovering costs.

Figure 3: Freeway Traffic Typical congested traffic on an urban freeway – I-80 in Berkeley, California. Residents of U.S. cities typically require automobiles to experience mobility. Note the externalities that the drivers are imposing on others such as air pollution and congestion. The left lane is for car-pooling – as marked by the white diamond – an attempt to address the congestion externality. Source: By User Minesweeper on en.wikipedia (Minesweeper) CC-BY-SA-3.0, via Wikimedia Commons

Traffic congestion is an example of what economists call externalities, the costs of an activity that aren't paid by the person doing the activity. Suburbanites who drive into the city every day don't breathe the polluted air produced by their cars; urban residents suffer that externality. People around the country who use gasoline derived from oil wells in the Gulf of Mexico didn't experience oil washing up on their beaches after the BP disaster in 2010. By charging the full cost of travel via taxes on gas or insurance, we could, for example, pay for children's hospitalization for asthma caused by the cars speeding past their neighborhoods. Or we could purchase and preserve wetland areas that can absorb the floodwaters that run off of paved streets and parking lots, keeping people's basements and yards drier. Not only would this help to deal with some of the externalities that currently exist, but the higher cost of gas would probably lead us to focus on accessibility rather than mobility, reducing overall demand.

The other way we can produce more sustainable transportation is to plan for accessibility, not mobility. Many transportation planners say that we've been using the predict and provide model for too long. This means we assume nothing will change in terms of the way we travel, so we simply predict how much more traffic there is going to be in the future and provide roads accordingly. Instead, we should take a deliberate and decide approach, bringing in more people into the planning process and offering different options besides more of the same. Some of the decisions we can make to try and change travel patterns include installing bike lanes instead of more parking, locating retail development next to housing so people can walk for a cup of coffee or a few groceries, or investing in transit instead of highways.

Figure 4: Traditional Plaza A traditional city center in Piran, Slovenia. The region around the square is mixed use, with buidlings serving both residential and commercial functions. The square is highly accessible to residents. Source: Plamen Agov studiolemontree.com.

For example, the school district in Champaign, Illinois, is considering closing the existing high school next to downtown, to which many students walk or take public transit, and replacing it with a much larger facility on the edge of town, to which everyone would have to drive or be driven. The new site would require more mobility on the part of nearly everyone, while many students and teachers would see their accessibility decrease. As gas prices continue to rise, it will cost the school district and parents more and more to transport students to and from school, and students will be more likely to drive themselves if they have access to a car and a driver's license. Putting the new school in a more accessible location or expanding the existing one would keep the school transportation system from becoming less sustainable.

You may have noticed that these proposed changes to increase transportation sustainability aren't really things that one person can do. We can certainly make individual choices to drive less and walk or bike more, to buy a more fuel-efficient car, or to use telecommunications instead of transportation. In order to make significant changes that can reduce overall energy usage and emissions production, however, the system itself has to change. This means getting involved in how transportation policy is made, maybe by attending public meetings or writing to city or state officials about a specific project. It means contacting your Congressional representatives to demand that transportation budgets include more money for sustainable transportation modes and infrastructure. It means advocating for those who are disadvantaged under the current system. In means remembering that transportation is connected to other activities, and that focusing on how the demand for transportation is derived is the key to making and keeping it sustainable.