Transportation in the US is the largest source of greenhouse gases — contributing over a quarter of the nation’s emissions in 2016. While transitioning to electric vehicles and increasing mass transit are necessary steps in addressing climate change, there is one simple action that is often overlooked. It is the lowest of low hanging fruits; the equivalent of an environmental magic pill because it can be done instantly at no cost with minimal negative side effects. The simple trick is slowing down. This obvious recommendation has been made many times before and most people and policymakers ignore it. But before speeding on, consider the following.
The science and history
Most people think of their car’s engine (if at all) in terms of its power. Power is a selling point and American automakers have almost doubled average horsepower since 1980. Why? Because power does two things. Power is the rate at which energy is expended, so a powerful engine has good acceleration — something that drivers like. Furthermore, transportation on Earth occurs in a fluid (either water or air) that must be pushed out of the way in order to maintain a constant speed. The power required to overcome the drag of air resistance increases by the cube of the speed. That is, if a vehicle speed doubles, the engine will need to provide roughly eight times as much power to overcome drag. So as speed increases, the energy expended to maintain that speed increases dramatically.
The real world is a bit more complex. For example, most gas-powered automobiles reach peak efficiency around 30 to 55 mph depending on the model. Inefficiency at lower speeds is due primarily to characteristics inherent to internal combustion engines. Electric-powered cars have fewer low-speed issues and typically have peak efficiencies in the 10 to 30 mph range. At higher speeds, the drag from wind resistance is the overwhelming factor for all vehicles.
In reaction to the 1973 OPEC oil embargo, the self-explanatory 1974 National Maximum Speed Law set US highway speeds limits at 55 mph to take advantage of the peak efficiency of internal combustion engines. Due to political unpopularity, the law was repealed in 1995. Automobile manufacturers internalized the lesson of that repeal — if you want to retain market share, design more fuel efficient vehicles, but make sure they can go fast. When Martin Eberhard and Marc Tarpenning first envisioned Tesla Motors (the first successful American car company since WWII), they wisely started with a sports car. Drivers may appreciate efficiency, but they get excited about speed.
The 55 mph speed limit law didn’t save as much gasoline as expected due to poor adherence, but a return to lower speed limits could be more easily enforced today. While speed cameras are widely used in Europe, automated ticketing systems remain unpopular and even prohibited in some US states. Fortunately, less confrontational methods are available. Many newer vehicles can be equipped with intelligent speed adaptation technologies and almost all cars have electronic speed limiters that could be set lower. The energy savings would be substantial. Just reducing speed 5 to 10 mph can increase fuel efficiency by 10% — a feat much harder for automakers to achieve through other technological means.
So how fast should we go?
Slowing down saves energy, but not always when transporting time-sensitive objects. For example, perishable foods require additional energy for refrigeration during transit to avoid waste. Yet, faster travel also creates wasteful opportunities. Faster travel allows perishable foods to travel a greater distance and reach a larger market — a trade-off of energy for economic benefit. Likewise, fast air travel encourages unnecessary globetrotting which increases individual carbon footprints. (Ironically, this includes climate scientists who frequently attend international conferences.)
There is also the consideration of necessity. While most people don’t need fresh fruit from other countries, a few activities do require high speed. For example, if you want to explore the solar system, the earth’s escape velocity is about 25,000 mph. However, most transportation doesn’t have minimum speed requirements. Instead, minimum acceptable speeds are set by human impatience.
A recent idea in high-speed travel is the Hyperloop system. The notion of a vacuum tube train — where people and freight travel through tubes at normally supersonic speeds with minimal air resistance — has been around for at least a century. Previously, the idea never made it past the realm of science fiction or engineering studies, but with the support of Elon Musk we now have Hyperloop test tracks and engineering competitions. The allure is two-fold. The operational energy use of such a system would be much less than the current air transportation system. More importantly, it would also be even faster. Hyperloop is not a complete replacement for airplanes since it’s not feasible for transoceanic travel. However, about 60 percent of air travel is domestic and over 4 billion passengers traveled by air in 2017. That is a lot of potential customers.
However, critics of Hyperloop have noted its substantial infrastructure requirements which makes it a very costly system to build and maintain. Life cycle assessment research suggests that, when total infrastructure and operational costs are included, there are no clear environmental winners among various transportation systems (car, rail, air, etc.) without considering the most important factor — ridership rates. For example, a full car is about five times more efficient per rider than a single-occupancy vehicle (hence, the wisdom of carpooling). If ridership is the important variable, then an infrastructure-intense system is hindered by the difficulty of constructing a network large enough to be convenient.
In a similar case, Oxford transportation experts Moshe Givoni and David Banister argued that high-speed trains have actually stalled the expansion of rail service in Europe. The problem is that high-speed trains were originally intended to improve rail capacity, but the discussions quickly became fixated on the maximum speed of the trains. The expense of building these specialized high-speed routes came at the expense of conventional rail systems which ended up hurting overall ridership.
The underlying problem is that high-speed trains are only fast if there are few stops. Therefore, the train stations must be spaced far apart which makes them less accessible. Since travelers care about door-to-door time more than station-to-station time, high-speed trains provide considerable time benefit only for specific trips — usually between a pair of large cities. This same effect can be seen in the airline industry where the centralized hub-and-spoke network has transitioned towards a more decentralized and convenient network of direct routes. As a result, Airbus recently announced it would end production of its largest and most expensive plane, the A380, which was intended for hub-to-hub flights.
Presumably, a Hyperloop system would have the same effect of concentrating benefits around specific routes. So, unless you can build a very extensive network, like China’s high-speed rail system, the overall systemic results are modest compared to the expense. Given the unreliable cost estimates for Hyperloop, the prohibitive cost of long-distance transit tunnels, and the cost overrun record of US high-speed rail, very high-speed ground transportation in the US will likely remain a future goal for some time. Alternatively, investing in “higher speed rail” that improves the existing US rail system (designed primarily for slow-moving freight) accomplishes much more than trying to recreate the European high speed passenger rail system. In essence, the most workable vision of a low-carbon transportation system is not the Mars-ready Hyperloop, but a massive interconnected network of medium-speed rail and electric buses.
We should take a lesson from the Concorde, the supersonic airliner that provided transatlantic flights from 1976 to 2003. The high cost of development could not be recouped by the British and French aviation companies that built the plane and the debt eventually had be absorbed by their respective governments. During its entire operating life, the Concorde was expensive to run and was considered a luxury service for the rich. Yet, the technical challenges of overcoming the aerodynamic drag and heat of supersonic speed made creating a luxurious travel experience that much harder. It worked when speed was considered the primary luxury, but failed as first-class conventional air travel became more sumptuous and lucrative over time. Despite occasional plans for new supersonic commercial planes, it seems that we may have hit “peak speed” in the transportation sector. If we really care about efficiency, perhaps more opportunities lie in the opposite direction (such as slower hybrid airships that can carrying freight or tourists to remote locations).
It’s easy to get lost in the possibilities of new technology, but we may be working towards the wrong goal. For the last century, the unquestioned assumption was that maximizing speed was the most important factor. However, modern communications technology has increased the productivity of travelers which decreases the “wasted” time of slower travel. Coupled with steep carbon emissions reduction goals, the time/energy trade-off is swinging towards slowing down.
Of course, slower transport has some potential negative consequences — such as less exotic produce and tourism, but slowing down also has ecological and aesthetic benefits. For example, one result of our modern fast transportation system is the loss of remote wilderness. It’s estimated that only 10% of the world’s land area is more than 2 days travel from a city. A slower world becomes a “bigger” world again.
A slow transportation movement may seem like a fringe idea, but only if we fail to question the outdated imperative for speed. As we reach towards a sustainable future, we should embrace both new technology and simple solutions. Ironically, one of the fastest ways to reduce the carbon footprint of transportation is to slow down.