My assumption is that this is intended to make a good first impression with the public - even if making the cars more risk-taking would result in a lower accide

My assumption is that this is intended to make a good first impression with the public - even if making the cars more risk-taking would result in a lower accident rate, an accident where the driverless car is technically "at-fault" can be very damaging to the overall project. Googlers working on the driverless car have explicitly talked about cautiousness as a dial that they've turned all the way up - not as an inherent requirement of the technology, but as a conscious decision.