A group of 30 companies has spent at least $16 billion on developing fully self-driving cars over the past few years—and so far they have little, if any, revenue to show for it. But billions more will likely be needed before the technology is ready for primetime.

Just three companies spent half of that money—Alphabet’s Waymo, GM’s Cruise and Uber, according to our calculations. Four other companies, including Apple, Baidu, Ford and Toyota, spent most of the rest. Only a few of those—most prominently Alphabet, Toyota and Apple—have the kind of immensely deep pockets that guarantee they can stay the course for the long run. And Apple has already pulled back on its investment.