Level 0: No automation - The driver is in complete and sole control of the vehicle.
Level 1: Driver assistance - The vehicle has one or more control functions e.g. cruise control.
Level 2: Partial automation - The vehicle has at least two primary control functions e.g. adaptive cruise control, lane centering.
Level 3: Conditional automation - The driver is only required for occasional control as the vehicle can perform all safety-critical functions under certain conditions e.g. the Google car.
Level 4: High automation - The vehicle can perform all safety-critical driving functions in all but a few environments. RAC Intellibus™ is a level 4 automated vehicle!
Level 5: Full automation - The vehicle can perform all safety-critical functions and other than setting the destination, no driver intervention is required.