At present, there is no practical means for testing the safety of AVs prior to widespread use...

According to the Bureau of Transportation Statistics, there were about 35,000 fatalities and over 2.4m injuries on American roads in 2015. While these numbers sound high, given that Americans drive three trillion miles a year, the accident rates are remarkably low—1.12 deaths and 76 injuries per 100m miles. Because accidents are so rare (compared with miles traveled), autonomous vehicles “would have to driven hundreds of millions of miles, and sometimes hundreds of billions of miles, to demonstrate their reliability in terms of fatalities and injuries,” says Nidhi Kalra of RAND Corporation, a think tank in California. At present, there is no practical means for testing the safety of AVs prior to widespread use. For many, that is a scary thought.

That's a lot of miles. Also, the article has good examples for each of the different levels (Level 1-5) characterizing the capabilities of autonomous vehicles.

Via The Economist