Commercial trucking is the first market that will be overturned by self-driving cars. More predictable routes, do not have to deal with passenger misbehavior, higher total profits to absorb early-on technology costs associated with creating the self-driving vehicle (LIDAR costs and whatnot).

Google has other realms that could benefit from self-driving cars, not just ride-sharing/taxi. Why have separate fleets of vehicles if you can have one that does taxi services, shopping deliveries, and data collection (street view) at the same time?