Distance is the metric used so far but also isn't appropriate for comparing self-driving cars to human driven cars. The human driven bucket contains all miles driven, highway cruise control, light snow, heavy rain, tricky merges, etc. The self-driving metric is only the easiest possible miles. Overtime those miles will expand and harder scenarios will be incorporated but to really know if self-driving cars are safer we need apples to apples comparisons which is going to require matching humans vs robots on miles driven and a categorization of those miles, maybe a count of tricky unexpected scenarios as well.