Of course, we also know that at least one self-driving car was involved in a crash when allegedly under manual control.
Naturally, we would expect these cars to be crash free or they should have been removed from traffic. Google stated that the product team has been "encouraged by this progress, but there’s still a long road ahead." Chris Urmson, in charge of driverless car engineering added: "To provide the best experience we can, we’ll need to master snow-covered roadways, interpret temporary construction signals and handle other tricky situations that many drivers encounter. As a next step, members of the self-driving car team will soon start using the cars solo (rather than in pairs), for things like commuting to work."
In addition to the current Prius vehicles. Google has recently added a Lexus RX450h SUV to its fleet. The car is supposed to take the technology on "different terrain", even if it is somewhat clear the Lexus RX is not exactly the vehicle that you will voluntarily take on terrain other than streets. Perhaps the Google crew was simply tired of the Prius?
In case you are interested, the National Highway Traffic Safety Administration said that there are 2.5 property damage accidents for every 1 million highway miles that are (manually) travelled. There was no information on accident rates on all U.S. streets.