Life in the automatic lane
Level 0
An automated system issues warnings, but has no vehicle control.
Level 1: hands-on
Driver and automated system share control of the vehicle, such as cruise control (driver controls steering, automated system controls speed) and assisted parking (automated steering, manual speed). The driver must be ready to retake full control at any time.
Level 2: hands-off
An automated system takes full control of accelerating, braking and steering. The driver must monitor and be prepared to intervene at any time. Hands-off should not be taken literally.
Level 3: eyes-off
The driver can safely turn their attention from driving; for example, they can text or watch a movie. The automated system will handle situations that call for an immediate response, although the driver should be prepared to intervene in a particular time, specified by the manufacturer, when prompted by the vehicle.
Level 4: mind-off
Same as Level 3, but no driver attention is ever required - they can safely go to sleep or leave the driver's seat. Self-driving is supported only in limited areas (geofenced) or under special circumstances, like traffic jams. Otherwise, the vehicle must be able to safely abort the trip, such as by parking, if the driver does not retake control.
Level 5: wheel optional
No human intervention is required at any time, like a robotic taxi.
Source: Sae International
