Level 0: Vehicles having level 0 autonomy are completely manual, with a human driver. Most old cars belong in this category.
Level 1: Vehicles with level 1 autonomy will have a human driver, but they will also have a driver assistance system that can either automatically control the steering system or acceleration/deceleration using information from the environment. All other functions have to be controlled by the driver.
Level 2: In level 2 autonomy, the vehicle can perform both steering and acceleration/deceleration. All other tasks have to be controlled by the driver. We can say that the vehicle is partially automated in this level.
Level 3: In this level, it is expected that all tasks be performed autonomously, but ...
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