The U.S. Department of Transportation's National Highway Traffic Safety Administration (NHTSA) defines vehicle automation as having five levels:
No Automation (Level 0):The driver is in complete and sole control of the primary vehicle controls - brake, steering, throttle, and motive power- at all times.
Function Specific Automation (Level 1) Automation at this level includes one or more specific control functions. Examples included electronic stability or pre-charged brakes, where the vehicle automatically assists the driver when breaking in order for the driver to regain control of the vehicle or stop faster than by acting alone.
Combined Function Automation (Level 2): This level involves automation of at least two primary control functions designed to work in unison to relieve the driver of control of those functions. An example of combined functions enabling a Level 2 system is adaptive cruise control in combination with lane centering.
Limited Self-Driving Automation (Level 3): Vehicles at this level of automation enable the driver to cede full control of all safety-critical functions under certain traffic or environmental conditions and in those conditions to rely heavily on the vehicle to monitor for changes in those conditions requiring transition back to driver control. The driver is expected to be available for occasional control, but with sufficiently comfortable transition time.
Full Self-Driving Automation (Level 4): The vehicle is designed to perform all safety-critical driving functions and monitor roadway conditions for an entire trip. Such a design anticipates that the driver will provide destination or navigation input, but is not expected to be available for control at any time during the trip. This includes both occupied and unoccupied vehicles.