While obviously density improves the efficiency of almost any transportation system I don't understand why driverless vehicles would ever be less efficient than

While obviously density improves the efficiency of almost any transportation system I don't understand why driverless vehicles would ever be less efficient than traditional cars. Sure they will initially cost more due to additional sensors and computers there will also be saving from removing unneeded human controls. Lastly accident/fatality rates are several times higher in rural areas so the potential safety gains are larger.