The researchers came to their conclusions through a physics-based model that reflects the "general trends" of the industry. It considers factors like weight, battery energy and the lift-to-drag ratio.
This doesn't mean you'll never leave the ground. The economy tips in your favor beyond the 22-mile mark, since flying cars gain efficiency the longer they can cruise at speed (in the study, 150MPH). The value is particularly strong for fully-loaded vehicles making long trips. A 62-mile trip with four occupants would use 52 percent lower emissions than combustion engine ground cars, and six percent lower than ground-based electric models. Significant obstacles like heavy traffic or rivers could also make flying cars more eco-friendly.
Thankfully, the industry is already leaning in that direction. Companies like Airbus and Bell are working on flying taxis that would frequently take multiple passengers, fly in packed urban areas, or both. The main obstacles now are more likely to be social factors such as affordability, noise and public acceptance (including regulations). Flying cars could still become a practical reality -- they just won't be ubiquitous like you see in the movies.