However, by percentage far more accidents happen in cities (including minor scrapes while parking etc), and the average driver's miles are a mix of city and hig

However, by percentage far more accidents happen in cities (including minor scrapes while parking etc), and the average driver's miles are a mix of city and highway (perhaps around 50/50? Numbers for that are hard to find). The waymo/robotaxi driving is basically entirely city driving, so I think I think it's reasonable to say human accidents for that type of driving are higher, possibly nearly twice as high as the estimate you got.