OK, let's talk about interface. The major problem such interface has to solve - is how keep the driver aware enough of the road at times he needs to(legally, at all times) , while making it more FUN for him than regular driving, otherwise, why bother ?
That's why i believe entertainment in some form, is critical.
So maybe that siri personality is also built in such a way to enable all-time awareness by the driver, effortlessly ?
Or another option: maybe they use some kind of brain monitoring machine like eeg, and together with music put you in some kind of meditative state fit for driving, yet very enjoyable ? but doing so in a subtle way ?