Opinion: Flying cars should have a limited role in sustainable mobility
The Jetsons cartoon created a futuristic image of flying cars back in the 1960s. Flying cars are now in the prototype stage, with commercial deployment expected in the near future.
We recently studied the sustainability implications of flying cars and found that this rapidly evolving technology provides benefits in a niche role, but also could result in unintended consequences.
Before this technology comes to market, its full effects should be considered, including displacement of more efficient travel modes and promotion of urban sprawl.
Flying cars, formally known as vertical takeoff and landing aircraft (VTOLs), could contribute to sustainability by displacing single-occupant cars in areas where congestion makes it difficult to expand efficient mass transit systems.
Examples include trips from Irvine to Malibu or San Francisco to San Jose. VTOLs may also be beneficial for trips with geographic constraints that are removed by flying, like having to drive around a Great Lake from Detroit to Cleveland or Grand Rapids to Milwaukee, or where higher-emission regional jets are the prominent option.
The Center for Sustainable Systems at the University of Michigan’s School for Environment and Sustainability has focused on sustainability assessments of products and emerging technologies for over a quarter century. Transportation has been an important part of our research agenda since our inception. We have studied electrified vehicles, connected and automated vehicles, and now flying cars to help guide their deployment and enhance their environmental, economic, and social sustainability.
Last week, Nature Communications published our assessment of the energy consumption and greenhouse gas emissions of flying cars. Since then, our paper has received much attention.
We focused on comparing flying cars to ground-based cars across various deployment scenarios. This is an important comparison since light-duty vehicles – sedans, SUVs, and trucks – are currently the most common mode of transportation in the United States. However, the comparison to ground-based cars we made in the Nature Communications paper is only part of the story. From a broader mobility system perspective, it is also imperative to understand how flying cars could influence mode shifts (e.g. from light rail to flying cars), and if they are likely to contribute to urban sprawl.
Flying cars combine the capabilities of a helicopter for vertical takeoff and landing, with the cruise efficiency of a winged airplane. We studied electric VTOLs; although they produce zero emissions during flight, their batteries require electricity generated at power plants.
To determine their energy requirements, we developed a physics-based model to characterize generic VTOL design based on publicly-available data. Several aerospace corporations and startup companies—Airbus, Boeing, Joby Aviation and Lilium, for example—and agencies such as NASA have developed prototypes. There is no convergence on VTOL design yet, indicating that the industry is still in its infancy and that we now have the opportunity to incorporate sustainability as a primary driver into VTOL development, instead of it being an afterthought.
The results presented in our Nature Communications paper indicate the sustainability performance of flying vehicles is dependent on two factors: trip distance and occupancy.
We find that for trips longer than 60 miles, a fully-loaded 4-occupant VTOL results in fewer emissions than average occupancy ground-based cars. It is important to note that a single-occupant VTOL never outcompetes a single-passenger electric ground-based car.
We strongly discourage VTOLs as a means of transit within urban centers. Flying a short distance in a VTOL is inefficient because of the energy-intensive takeoff and hover phases of flight, and the relatively short time spent in the more efficient cruise mode. Not only would energy use increase compared to ground-based cars, but urban deployment would increase noise, visual disturbance, and aerial traffic concerns.
We also caution against deployment that leads to unintended consequences. Public transit systems such as light rail are the most efficient modes of transportation. Buses can also perform well when they have high occupancy. If VTOL deployment shifts travel from these modes, it will be unfavorable from an environmental perspective. For reducing environmental impacts of transportation, investment may be best directed towards improving public transit and increasing ridership levels.
VTOLs could also encourage less dense patterns of development. If VTOLs make travel more convenient, they could induce travel demand and encourage people to live further from work. Strategic policies and regulations that would constrain VTOL deployment to the types of trips where they are the best option may be effective tools to mitigate this effect. Urban design focusing on mixed-use and high-density development could also encourage more people to live closer to work and reduce time and distance spent commuting, resulting in reduced environmental impacts.
So, what is the future for flying cars?
We want to emphasize to manufacturers and service providers that when VTOLs are launched commercially, they should only be deployed in certain scenarios.
While concluding that high-occupancy VTOLs can have a niche role in sustainable mobility for long-distance trips, we want to actively avoid unintended consequences like urban sprawl and increased vehicle miles traveled. Further, VTOLs should complement public transit for congested, indirect, or long-distance routes, and should do so in an equitable manner that increases access.
In conclusion, our study comparing flying cars to ground-based cars is only one chapter of the story; the rest remains to be written.
As a society, we must be thoughtful about what comes next. It is important to recognize that any aerial transportation has an inherent disadvantage owing to the laws of physics. However, we recommend further detailed analyses of flying cars compared to other transportation modes for identifying other niche applications where they provide an environmental benefit.
Given the grand challenge of climate change and the urgent need for the deep decarbonization of transportation, we hope that our recommendations can help guide sustainable development of flying cars and constrain their use solely to cases that enhance environmental, economic, and social dimensions of future mobility.
Greg Keoleian is the Peter Wege professor of sustainable systems in the School for Environmental and Sustainability, professor of civil and environmental engineering, and the director of the Center for Sustainable Systems at the University of Michigan.
Jim Gawron is an Erb Institute MBA/MS candidate at the Stephen M. Ross School of Business and the School for Environment and Sustainability, and a graduate student research assistant within the Center for Sustainable Systems at the University of Michigan.
Akshat Kasliwal is an MS candidate at the School for Environment and Sustainability, and a graduate student research assistant within the Center for Sustainable Systems at the University of Michigan.