The startups and investors that have sent hopes soaring for “flying cars” could be in for a rough landing, in more ways than one.

Hundreds of companies, new ones and legacy aviation players alike, are working on such vehicles—also called air taxis or eVTOLs (short for electric vertical take off and landing). Five such startups have gone public in the past 12 months. They are trying to shape a near future in which taking a flying cab is an economically viable alternative to taking a terrestrial one.

The biggest stumbling block to that sci-fi vision, though, is rather down-to-earth: Flying-car companies haven’t figured out how to site, permit and construct enough places for their vehicles to land and take off to allow a workable business model for making and operating sky taxis.