Self-driving cars won’t dominate American roadways for decades given millions of older vehicles consumers own, the top U.S. federal highway safety regulator said.

Traditional auto makers, suppliers and Silicon Valley giants and startups alike are racing to develop autonomous vehicles. But there are roughly 250 million cars and trucks currently on U.S. roadways that are more than a decade old on average, highlighting potentially long lead times before vehicles that predominantly drive themselves proliferate, Mark Rosekind, head of the National Highway Traffic Safety Administration, said Tuesday.

“For the next 20 to 30 years at least, we will likely have a mixed fleet of different levels of automation,” Mr. Rosekind said before a House Energy and Commerce subcommittee. Ubiquitous fully self-driving cars “are years off,” he said.

Auto makers are already deploying semiautonomous technologies that allow cars to perform driving functions themselves under limited circumstances, including automatic emergency brakes and adaptive cruise control that changes speeds based on traffic patterns.

Autonomous vehicles that don’t require human intervention remain in testing stages, with companies eyeing ride-sharing fleets as initial laboratories for rolling them out to consumers. Uber Technologies Inc. earlier this year began letting some consumers hail autonomous taxis manned by backup drivers and engineers in the front seat to ensure safety.