Take a guess, when did smartphones reach 50% penetration [in the U.S.]? In 2012, according to
this overview. If you want, you can put the beginning of smartphone at 2007 but you could also mark 1996 as the start when the first Nokia Communicator came out. Which would put the 50% milestone at 16 years.
So one question is where are we in regard to autonomous cars? I'd say the Nokia Communicator moment in that regard was only a few years ago (adaptive cruise control + lane control). Then there is the question how long does it take from the iPhone moment (the first mass market implementation that is actually good) to the 50% marker? Here, you have to take into account how long the life cycle of a product is, ie, how frequently consumers replace it. For phones that is probably in the range of 2 to 3 years. For cars that is more like 5 to 8 years, ie, more than twice than that. Which would stretch to iPhone to 50% penetration rate from five (2007 to 2012) to more than ten years. Thus even if we had the first fully autonomous car in 2018, that would put the 50% mark at about 2020.
And most new technologies have noticeably slower adoption rates. If we take the PC and use the Mac from 1984 as the mass market start, it took until 2000 to reach 50% penetration or 16 years compared to the 5 years of the smartphone (and the longer life cycle of PCs does not fully account for this). And that is before we consider that the regulatory burden (due to the much higher danger coming from cars compared to PCs or smartphones) will slow things further for autonomous cars.