Even Google's own data isn't perfect. A couple of years ago someone even tried to sue Google for "getting her on a highway" and then having an accident. I think she was walking, and obviously she can't blame Google for not noticing the highway!
But with self-driving cars, it's different. A car landing on a highway, when it's not supposed to be there, and then getting into an accident, is not good news. So that's why I think accurate map data will be very important for this.
Mapping data will be important, but not to prevent accidents. Maps will not be 100% accurate, so even with perfect maps, cars will end up on roads they think aren't highways while in fact, they are. There also will be newly one-way streets that aren't in the mapping database yet, roads closed due to construction or accidents, etc. Yes, both will be rare, but multiplied by millions of cars (necessary to get the 'worth trillions' part)? Not so.
So, let's say you are driving a vehicle that isn't allowed on the highway but incorrectly takes one. How do you think maps will help with the "and then getting into an accident" part?
But with self-driving cars, it's different. A car landing on a highway, when it's not supposed to be there, and then getting into an accident, is not good news. So that's why I think accurate map data will be very important for this.