Autonomous – or self-driving – vehicles are those in which operation of the vehicle occurs without direct driver input to control the steering, acceleration and braking, and are designed so that the driver is not expected to constantly monitor the roadway while operating in self-driving mode.
The U.S. Department of Transportation's National Highway Traffic Safety Administration defines vehicle automation as having five levels:
Level 0 – No-Automation. The driver is in complete and sole control of the primary vehicle controls – brake, steering, throttle and motive power – at all times.
Level 1 – Function-Specific Automation. Automation at this level involves one or more specific control functions. For example, electronic stability control or pre-charged brakes, where the vehicle automatically assists with braking to enable the driver to regain control of the vehicle or stop faster than possible by acting alone.
Level 2 – Combined Function Automation. 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 this is adaptive cruise control in combination with lane centering.
Level 3 – Limited Self-Driving Automation. 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.
Level 4 – Full Self-Driving Automation. At this level, 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
