1) Average driver drives ~13.5K miles/year (http://cars.lovetoknow.com/about-cars/how-many-miles-do-amer...) with an average speed of ~32 mph (http://www.ridetowork.org/transportation-fact-sheet). This translates into 420 hours/year. Or a person becoming an expert driver by the age of 40, if we accept the "10,000 hours rule"...
2) But the Gladwell's "10,000 hours rule" was proven wrong (http://journals.sagepub.com/doi/abs/10.1177/0956797614535810). For example, if we follow FAA that recently changed the rules to require 1500 hours for ATP (i.e. airline pilots) license, then you are talking about 3.5 years (for average driver) vs. 9 months (for taxi driver). Not that big of a difference.
This means that on average the US driver becomes an "expert" by the age of 21-22. This actually matches quite well with the insurance company rates that drop drastically at about this age.
Quick calculation: Lets think about people who drive their whole lives, not city folks. Lets think about work hours driving as it's likely to be the most by quantity, and vacation or pleasure driving should be roughly equivalent.
30 minutes each average way 5 days a week starting at 16, so by 40 they've driven 5/7365 = 260hrs / year. 260 24 years = 6240 hours.
8 Hours a day, 5 days a week, with salaried level vacations is 2000 hours a year. Many drivers are likely to work way more than that. Anyway, 6240 / 2000 = 3.12 years. That means that if you started driving at 18, you'd have the same hours driving as a 40 year old.
This also means that by the gladwell definition of 10,000 hours, you don't become an expert driver till 54.5 years.
However a pro driver becomes and expert after 5 YEARS of driving.
This also means that only 22% (those over 55) of america is likely to be an expert driver. http://www.infoplease.com/us/census/data/demographic.html
If HN population is the same as americans (it's likely skewed older), then 78% of hacker news isn't a gladwellian "Expert".
So self driving cars don't need to be better than most people they need to be better than a group of people who are very likely to be experts.