Your 1 accident per 500k miles stat is an average across urban and rural areas. But accident rates in urban areas are likely significantly higher than in non-urban areas, due to higher vehicle density/congestion, despite the fact that vehicle accident fatality rates are higher in rural areas[1]. Urban areas tend to have a lot more low-speed no-injury or mild-injury collisions. When you account for all of those, AVs are in shooting distance of human driver accident rates in urban areas. Also, the accident rates are much higher for drunk, sleepy, or distracted drivers, so to the extent AVs reduce those types of driving, they bring the overall accident rates down much further.
