It started up on Friday, when Slovakia-based startup AeroMobil announced it was ready to take pre-orders for its vehicle that’s set to arrive in 2020. Kitty Hawk, the company that’s financially backed by Google founder Larry Page, followed that news by unveiling the first footage of its own prototype aircraft. A day later, Uber kicked off its Elevate Summit, a three-day conference to explore the company’s vision for “urban air mobility” in Dallas.

Flying cars are no longer the stuff of science fiction, but some designers and engineers aren’t on board. Here are three chief concerns that could hold back the widespread adoption of airborne vehicles.

Air Traffic Control

The Federal Aviation Administration (FAA) oversees all civil aviation in the United States, and while the agency has approved test flights for several flying car prototypes over the past two years, it would require a significant undertaking to allow consumers to pilot their cars throughout the skies.

Flying cars can theoretically avoid highways or bridges, but the U.S. would need a brand new air traffic control system in order to turn that into a reality. The FAA has already begun work on a parallel system to safely direct delivery drones, but similar measures have not yet been taken for flying cars. “Several areas need further research, particularly identifying the operational risks, making sure the automation that ‘flies’ the autonomous vehicle is safe, and how the automation will interact with the air traffic control system,” an FAA spokesperson wrote to Fortune in an email.

Jan Achenbach, professor emeritus at the McCormick School of Engineering at Northwestern University, said that he couldn’t imagine flying cars functioning the same way in an urban environment like Chicago, where infrastructure could be difficult to establish and maintain in part to the number of people traveling on a daily basis. “You’d have to design ‘flyways’ in the sky that follow regular roads from above as a secondary system,” he said. “But it seems quite difficult. As soon as [flying cars] take off they are subjected to the rules off the FAA, and there’s already enough problems with little drones, yet alone [car] traffic.”

High Costs

AeroMobil said its commercial flying car will pre-order for 1.2 to 1.5 million euros, or roughly $1.3 million to $1.6 million. The first footage of the Kitty Hawk Flyer — which has no wheels — has been described as “an expensive toy for your next trip to the lake.” So that begs the question: will flying cars be a realistic mode of transportation for middle-class consumers?

Clifford Winston, a senior fellow in the Brookings Institution’s economic studies program, said it ultimately comes down to the amount of flying cars that can be supported in the sky; specifically, if infrastructure would be able to accommodate a sufficiently large number of cars that can operate at a scale that lowers the cost for the everyday consumer.

“Think about how many planes we have flying around — in the thousands — you’re not going to be able to bring that scale down with thousands of cars – it would have to be millions,” Winston added. “We don’t even have the kind of scale to make planes affordable for most people.”

There are also questions over whether the batteries that power flying cars are a strong enough source of energy, which would be the case for Uber’s flying car technology. “The biggest issue with the [Uber] Elevate manifesto is that it assumes the existence of a truly amazing battery with a phenomenal energy density,” aviation consultant Richard Aboulafia told NPR. “We’ve been striving for this for the last century. You’d need the kind of reliability that puts you in the air and keeps you there safely. This isn’t a Tesla — you can’t just pull over to the side of the road.”

Safety

While airplanes are considered one of the safest methods of travel, placing an additional number of heavy vehicles in the sky is cause for concern for both flying car pilots and bystanders on the ground. “We don’t want flying cars dropping out of the sky on people’s houses when somebody gets sick or has a heart attack,” Robert J. Gordon, professor of the social sciences at Northwestern University, said.

Unlike airline pilots who suddenly get sick, those behind the wheel of a flying car presumably won’t have the contingency of a co-pilot. In some cases, airline pilot Chris Manno previously told Mashable, there are also additional reserve pilots aboard a flight for longer trips or are simply traveling to other assignments. Manno is an American Airlines captain in the Dallas/Fort Worth area.

Gordon added that drivers of flying cars would have to pass frequent physical exam tests similar to private pilots — not ideal for commuters hoping to use their flying cars to get to work or school on a daily basis. He then looped back to the issue of air traffic control, but looked at it from a more local level.

“You can’t have flying cars going about like cars on roads, because there are no roadways, red lights or green lights in the air,” Gordon said, adding that flying car drivers would need to pass tests in constant radio communication that would require a pilot licenses in addition to a standard drivers license.

Some would argue that widespread flying cars, and particularly in cities, would also necessitate autonomous technology. But with self-driving cars still getting into accidents — at least one of which was fatal — it could be a long time before flying cars take off.