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| Fig. 1: E-scooter from the company Lime, pictured on a sidewalk in Oakland, California. (Source: Wikimedia Commons) |
Walking around the streets of major U.S. and European cities, you will likely see electric scooters strewn around here and there, seemingly unlocked and unattended, as in Fig. 1. Only a few years ago, this would likely have raised eyebrows, but in the year 2020 they are a familiar part of the cityscape in the same way bicycle racks or parking garages are. They typically belong to one of a few different ride-sharing companies, Lime and Bird being two of the most prominent ones, that maintain a large fleet of scooters that their customers are able to share with each other. [1] These companies provide smartphone applications that users can log into in order to find a scooter parked nearby, activate it, and pay for their usage once they are finished.
They have only been around for a few years, but the response to e-scooter ride sharing has for far been mixed. Proponents for these ride sharing companies, not least of all the companies themselves, point out a number of benefits with e-scooters, from helping reduce global emissions by giving people a cleaner alternative for city transportation than driving a car, to improving urban quality of life by decreasing noise and traffic. Detractors, on the other hand, might argue that they endanger pedestrians and that insofar as they are substitutes for walking or biking, they actually add to cities' net carbon emissions. [2]
Electric motors in general are more energy efficient than combustion engines, meaning that they are able to convert a higher percentage of their input energy into useful work. [3] In addition to this, e-scooters are far lighter compared to a car, which means that a higher percentage of the energy spent goes towards transporting the actual passenger, rather than the vehicle itself, than would be the case for a car.
The smaller size of e-scooters also has advantages in terms of their construction. As the average e-scooter only weighs around 28 pounds compared to the average car weighing in at 4,100 pounds; the amount material and energy required to construct an e-scooter is a fraction of that required to construct a car. [4] Unlike the motor efficiency differential vis-a-vis combustion engines, this holds true even when we compare the e-scooter to an electric car.
Roughly half of the lifetime emissions of an e-scooter comes from the raw material extraction and manufacturing process. [5] How much these average out to on a per-mile basis then depends on how many miles the e-scooter actually ends up traveling. This of course depends on both its utilization rate and its lifetime. While the scooters theoretically can last two years, studies have shown that they last only a month or two on verage. [5] Similarly, a study found that the lifetime emissions per mile of e-scooters is quite sensitive to its utilization, as measured on a miles-per-day basis. [4]
Another estimated 43% of the total lifetime emission of an e-scooter comes from the emissions associated with collection, charging, and distribution. [4] E-scooters are left on the sidewalks, and when they have run out of charge, they are picked up by regular trucks and driven to centralized charging locations. After completing charging they are redistributed around their city. Depending on how this collection and redistribution is done, the per-mile emissions of an e-scooter vary dramatically. [4]
Lastly, an important question is what the e-scooter is substituting for. By running Monte Carlo simulations over the range of possible values for the above factors (e-scooter lifetime, collection method, etc.), one study found that in general, when a personal automobile is the alternative mode of transportation, using an e-scooter almost always leads to lower emissions. [4] However, if the alternative is, for example, a bus with high ridership, or a personal bicycle, an e-scooter will very likely lead to higher emissions. [4] Studies done on e-scooter usage suggest that far from all e-scooter usage replaces car rides. In one study, only third of e-scooter rides replaced car rides. [3] In the other two thirds of all uses, the e-scooter is a substitute for e.g. walking, biking, or riding public transit, all of which are less emissions-intensive per mile than the e-scooter. [4]
Electric scooter ride sharing is often thought of as a substitute for cars for "last-mile" trips - trips less than a mile in length. It is estimated that only in the U.S., car trips shorter than a mile combine for a total of around 10 billion (1010) miles per year. [6] Assuming this number and an average of 20 mpg for the cars driving, this amounts to 1010 mi / (20 mi gal-1) = 5.0 × 108 gallons of gas each year. [6] With the energy density of gasoline being around 1.32 × 108 J gal-1, this comes out to a total of around 6.6 × 1016 J each year. [7] If the total yearly energy budget of human civilization is 5.84 × 1020 J, we get that the energy spent on car trips shorter than a mile in the United States only accounts for around 0.01% of this total. [7,8] Of course, this was just in the United States, but assuming the same automobile use patterns for all countries in the world still bounds the total at around 0.26%. This effectively puts a very crude ceiling on how much of a difference e-scooter ride-sharing can make. Even granting generous assumptions of the scooters' utilization rate, lifetime, and so on, if every car ride shorter than a mile was replaced by an electric scooter ride, we'd affect only 0.26% of the world's total energy usage.
In conclusion, whether or not e-scooters are environmentally friendly depends in large part on how long the scooter lasts, how it is charged and distributed, and above all, what the comparison point is. Riding an e-scooter one mile to work is almost always, from an emissions standpoint, preferable to riding your car there alone. However, you'd reduce your emissions even more by just walking or biking.
© Marcus Palsson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] K. Roof and C. Tse, "Scooter Startup Bird Discusses Going Public Via SPAC," Bloomberg News, 16 Nov 20.
[2] F. Yue, "Electric Scooters: Love or Hate Them? Here's What You Need to Know," USA Today, 14 Aug 19.
[3] L. D. D. Harvey, "Rethinking Electric Vehicle Subsidies, Rediscovering Energy Efficiency", Energ. Policy 146, 111760 (2020).
[4] J. Hollingsworth, B. Copeland, and J. X Johnson, "Are E-Scooters Polluters? The Environmental Impacts of Shared Dockless Electric Scooters," Environ. Res. Lett. 14, 8 (2019).
[5] J. Temple, "Sorry, Scooters Aren't So Climate-Friendly After All," MIT Technology Review, 2 Aug 19.
[6] A. Santos et al., "Summary of Travel Trends: 2009 National Household Travel Survey", U.S. Federal Highway Administration FHWA-PL-11-022, June 2011.
[7] M. Fischer, M. Werber, and P. V. Schwartz, "Batteries: Higher Energy Density Than Gasoline?" Energ. Policy 37, 2639 (2009).
[8] "Statistical Review of World Energy 2020," British Petroleum, June 2020.
