They start at Level Zero, which is an older, basic car with no autonomous functions at all.
Level One autonomy introduces technologies such as radar cruise control, auto braking, lane-keeping assistance and blind-spot monitoring – Level One AVs are now common.
Level Two autonomy combines various Level One features to provide the car with the ability to match the speed of the car in front up to a pre-selected speed, automatically brake if need be, and self-steer to keep in the lane – this isn’t too hard to achieve now.
Level Three autonomy is where the car starts to think for itself and can move out of lanes and navigate through traffic in an active way rather than the essentially passive/re-active Level Two mode. However, a human driver has to be ready to take control if need be in an emergency, something which the car industry sees as being very problematic going forward.
Level Four is full autonomous control (no human intervention at all), but only on roads and in locations specifically approved or designed for this purpose.
Level Five autonomy ups the ante further by also offering full autonomous control without road and location restrictions.