It seems to me that a fleet of driverless information-gathering cars would be a natural component of a world with driverless cars. Primarily they'd be used for

It seems to me that a fleet of driverless information-gathering cars would be a natural component of a world with driverless cars. Primarily they'd be used for continuously mapping roads for any potentially useful information or dangers, eg lane closures, construction, potholes, etc. From there, though, there's no reason you wouldn't naturally start gathering more information: traffic, weather conditions, strength of cell/wifi signals, ambient noise levels, and all sorts of other things. A highly distributed network of sensors could provide all sorts of interesting information. (Note that there's no reason that they'd even have to be empty cars. I see no reason why every car couldn't or wouldn't be decked out with sensors and reporting back to Google.)