Those of us that live in wealthier countries tend to travel a lot, whether for the daily commute or vacations to someplace tropical. The sustainability of our movement depends on two things, how far we go, and how we get there. Of course the most sustainable thing would be never go anywhere, but that would neither be pleasant nor practical in the modern age. What we can do is to stay aware of the miles that we are traveling, and try to minimize those travel distances where we can, especially when taking less efficient modes of transportation.
Transportation comes in right behind electricity for total greenhouse gas emissions, at 27% of all US emissions. The majority of those emissions come from all of the cars and light trucks out there on the road. If you drive an average car (like a full-size sedan or small SUV) the average number of miles (about 13,000 miles per US car) you are directly causing about 5 tons of carbon dioxide and its equivalents per year. Personal transportation is an area of our lives that we have a great deal of control over, and for most of us we could make different decisions that would lower the environmental impact of our travel by a huge margin.
I think that it makes the most sense to frame a discussion of modes of transport by first giving the best direct comparison that I could find, which is the amount of energy needed for one person to travel a given distance (see table below). These figures are averages drawn from here, here, and here, and are meant to be only rough approximations. For the vehicles, these are considered at average occupancy rates, so a car with 1.6 people in it, or an airplane with 84% of the seats full. While the fuel sources vary dramatically, from food powering a person to diesel for buses, one can convert these different kinds of energy into the same units to compare them. We use kilowatt hours (kWh) per kilometer here, but the comparisons between forms of transportation wouldn’t change if we used something else, like miles per gallon of gasoline (MPG). We’ll go through the various forms of transportation and weigh some of their pros and cons from a standpoint of sustainability while referring back to this table.
Biking and walking. These are big wins for both people and the environment. The energy used for getting around by person-power is food, and the human body is able to convert that food into motion very efficiently. We all need to eat anyway, and there are great health gains to be had from getting more exercise. In general, people are spending too much of their time sitting, and problems like obesity are continuing to rise throughout the world. If you live in a place that is conducive to walking or biking, make sure to get out there, and walk-ability and bike-ability should certainly be up there on the list of things to consider when looking at where to live and work.
Public transit. I was actually quite surprised to see the data on energy use for trains and buses, as it shows that one isn’t usually saving any energy by taking transit. Buses are much more efficient at moving people around than cars when full, but when all of the miles they travel when mostly empty are accounted for, they actually use more energy per person to move people around than cars. Keep in mind that these are US national averages, so denser cities with very high ridership rates may fare much better in analysis. So instead of being just about energy consumption, public transit is more about solving other problems, such as ensuring access to mobility to those who can’t afford cars or don’t want them, reducing congestion around cities, or reducing the amount of space devoted to parking lots and roads. Transit systems are a good and important part of the infrastructure of a city or a nation, but they have not been a means of reducing the amount of energy that we use to get around. If you can take transit to where you need to go you should certainly do so, but in terms of emissions you’d be doing fine in an efficient car and/or a carpool.
Air travel. Airplanes are roughly as efficient per mile at moving people around as cars, so that seems pretty good on the face of it. The problem is that we use up an awful lot of fuel in a very short time because of how quickly we are moving and how far we go. For example, take a person that lives in Ottawa (as I do), with daily commutes and other driving adding up to the national average amount of about 16,000 kilometers (10,000 miles). For a special treat, this person takes a one week trip to Hawaii. That round-trip flight is just as many miles, and therefore uses just as much fuel and causes just as much emissions, as all of the driving from the rest of the year combined. From a sustainability standpoint, this puts out a clear message – that we should be taking less flights. This doesn’t automatically mean that we are spending less time in distant places, as we could do things like taking fewer but longer trips. Frequent flyers, especially those traveling mostly for pleasure, need to at least become more aware of how much energy it is taking to get them around.
Personal cars and light trucks. It is a bit harder to make broad generalizations about our personal vehicles, as they vary so much in size and efficiency as well as how we use them. The biggest determinants of emissions are the efficiency of the vehicle, how many miles are traveled per year, how many people are typically in the vehicle, and fuel type. For gasoline and diesel vehicles, this translates directly into greenhouse gas emissions, while it is a bit more complicated for electric vehicles (see more in the next section below). The most appropriate vehicle really does depend on your needs. If you absolutely need to make long commutes alone, you should really be driving a small very high efficiency car, while if you spend most of your time shuttling around your family, then a much larger vehicle makes sense. One interesting point on occupancy is that the typical high efficiency car, think a Honda Civic, with one person in it has the same efficiency at moving people around as a full size pickup with two people in it. So sizing your vehicle to your needs really does make sense, but not to go bigger than your typical needs require. If you only need that huge vehicle once or twice a year, it makes better financial and environmental sense to own something smaller and more sensible and then occasionally rent the other vehicle(s) that you need. For the total number of miles traveled, the decision with the biggest impact is where you live in relation to where you work. Having a long car commute can easily double (or more) the amount of miles that you drive in your car and those miles are often alone, so living closer to work can have an outsized positive impact.
Future vehicle efficiency…we should electrify everything. In the table above, I included the Tesla Model 3 as an example of what the future holds, and what we ought to be moving towards now. Electric vehicles hold enormous efficiency advantages over internal combustion vehicles, as 80-90% of the energy in a battery is translated into moving a vehicle, while only about 25% of the energy in gasoline makes it to the wheels with most of the rest lost as waste heat. This is why that Tesla Model 3 fares so well in the table above, it is about four times as efficient as a comparable car with a gas engine. Of course the energy in the battery still has to come from somewhere and that is usually the local energy grid, so an electric car is only as clean as the grid energy. The good news is that electric cars now have less total emissions than gasoline cars almost everywhere, and our electricity grids are steadily cleaning up by phasing out coal and other fossil fuels while adding renewable forms of energy. Each year electric cars will pull further ahead of their gas powered cousins.
If electric transportation is so much better, why isn’t it already in widespread use? Electricity has been used for some mass transit with things like electrified overhead lines for decades, and there have long been experiments with electric cars, but the biggest impediment to widespread adoption has been the fact that batteries were always too expensive and too heavy. Modern lithium ion batteries have finally reached the point where the numbers make sense, and we are seeing the beginning of an explosion of electric vehicles onto the market. If there is an electric vehicle on the market that fits your needs, switching to electric for your next vehicle will have huge environmental benefits and will probably save you money in lifetime ownership costs.
Along with electric cars, we are seeing other forms of transportation also electrifying, and with similar gains in efficiency and reductions in emissions. Use of all electric city buses is growing quickly and will likely continue. Aviation will be much harder to electrify as batteries are still much heavier compared to liquid fuels, and weight is a much bigger factor when flying as compared to ground travel. It isn’t yet clear whether electricity can take over for all of our oil-based fuels, but it is going to rise, and rise quickly.
Embodied energy. Almost everything above has talked about the amount of energy needed to travel, but has ignored the energy needed to build the vehicles in the first place. This was for two good reasons. First, it kept things simpler. Second, the amount of energy that a car or other vehicle uses to travel over its lifetime is a great deal higher than the amount needed to build it, from mining the materials to the assembly at the factory. For a typical gas powered car, about 20% of the energy and emissions come from manufacturing, while 80% comes from the fuel it uses over its lifetime. For an airplane almost all of the emissions come from fuel, while for a very high efficiency electric car as much as half of the emissions are produced during production as it produces so little emissions once it is on the road.
It is now possible to find evaluations that try to take all of these impacts into account at once, called ‘lifecycle analyses’. These look at all the relevant aspects, from mining raw materials to recycling the product at the end of its lifetime, from oil in the ground to emissions out of the tailpipe. The Automotive Science Group produces an annual report that compares the vehicles on the market with a total lifecycle ‘environmental performance index’. Unsurprisingly, smaller, lighter, and more efficient vehicles, especially those with electric and hybrid drivetrains, come out as having the lowest environmental impact of all the vehicles available.