>Per mile incident rate is a measure only the companies care about
This is a very frustrating sentence to read. If the per mile incident rate of driverless cars is 1 and the per mile incident rate for normal cars is 2. What happens when you do a 1:1 replacement of all normal cars with driveless cars? The number of incidences halves. i.e the total number of traffic incidences halves.
You assume that a driverless mile replaces a human driven mile. I'm not sure that's a reasonable assumption. (I'd assume people would drive more miles if they don't need to worry about parking or paying for a human driver)
This is a very frustrating sentence to read. If the per mile incident rate of driverless cars is 1 and the per mile incident rate for normal cars is 2. What happens when you do a 1:1 replacement of all normal cars with driveless cars? The number of incidences halves. i.e the total number of traffic incidences halves.