The SAE Levels of Driving Automation come from SAE International’s standard J3016, “Taxonomy and Definitions for Terms Related to Driving Automation Systems for On-Road Motor Vehicles.” It is the reference almost everyone uses when they say a car is “Level 2” or “Level 4.” The taxonomy defines six levels by the role each actor, human driver and automated system, plays in the dynamic driving task: Level 0 (no driving automation), Level 1 (driver assistance), Level 2 (partial driving automation), Level 3 (conditional driving automation), Level 4 (high driving automation), and Level 5 (full driving automation).
The most consequential boundary is between Level 2 and Level 3. At Level 2 the human driver is still driving and must supervise constantly, even when the car steers and controls speed at the same time; at Level 3 the automated system performs the entire dynamic driving task within its limits, and the human is not driving while it is engaged. Levels 0 through 2 are framed as driver-support features, while Levels 3 through 5 are automated-driving features. The difference between Levels 4 and 5 is scope: a Level 4 system drives itself only within a defined operational design domain, while a Level 5 system, which does not yet exist commercially, would handle anything a human could.
The taxonomy is deliberately a description of capability, not a marketing promise, and SAE has revised it repeatedly to reduce ambiguity, because loose use of these terms, especially calling a Level 2 system “self-driving,” has real safety consequences.
For a general reader, the J3016 levels are the single most useful piece of vocabulary in the autonomy debate: they cut through hype by forcing a precise question, who is responsible for driving, and when.