  There is an urgent societal need to assess whether autonomous vehicles (AVs)
are safe enough. From published quantitative safety and reliability assessments
of AVs, we know that, given the goal of predicting very low rates of accidents,
road testing alone requires infeasible numbers of miles to be driven. However,
previous analyses do not consider any knowledge prior to road testing -
knowledge which could bring substantial advantages if the AV design allows
strong expectations of safety before road testing. We present the advantages of
a new variant of Conservative Bayesian Inference (CBI), which uses prior
knowledge while avoiding optimistic biases. We then study the trend of
disengagements (take-overs by human drivers) by applying Software Reliability
Growth Models (SRGMs) to data from Waymo's public road testing over 51 months,
in view of the practice of software updates during this testing. Our approach
is to not trust any specific SRGM, but to assess forecast accuracy and then
improve forecasts. We show that, coupled with accuracy assessment and
recalibration techniques, SRGMs could be a valuable test planning aid.
