Though autonomous cars are expected to transform parking lot development and increase neighborhood walkability as they take to the streets by 2020, one expert says driverless cars aren’t going to dominate US roads for at least two decades.

Mark Rosekind, a top US federal highway safety regulator, says US consumers already own millions of cars, and they aren’t quite ready to part with them. There are around 250 million cars and trucks on US roads that are more than a decade old on average, the Wall Street Journal reports, which means consumers tend to keep cars for at least 10 years.

Mark says for the next 20 to 30 years we can expect a mixed fleet of varying levels of automation, adding that despite massive efforts by automakers and Silicon Valley firms to make driverless cars a reality, it will still be a long time before they are anything near commonplace. [WSJ]