The Society of Automotive Engineers definitions divide vehicles into levels based on who does what, when. Generally:
- At SAE Level 0, the human driver does everything.
- At SAE Level 1, an automated system on the vehicle can sometimes assist the human driver conduct some parts of the driving task.
- At SAE Level 2, an automated system on the vehicle can actually conduct some parts of the driving task, while the human continues to monitor the driving environment and performs the rest of the driving task.
- At SAE Level 3, an automated system can both actually conduct some parts of the driving task and monitor the driving environment in some instances, but the human driver must be ready to take back control when the automated system requests.
- At SAE Level 4, an automated system can conduct the driving task and monitor the driving environment, and the human need not take back control, but the automated system can operate only in certain environments and under certain conditions.
- At SAE Level 5, the automated system can perform all driving tasks, under all conditions that a human driver could perform them.