There are several different levels of self-driving cars. Some vehicles are fully autonomous. Other cars use varying degrees of driver-assist technology while leaving the driver in control of the vehicle.
The National Highway Traffic Safety Administration classifies cars into levels based on their degree of autonomy:
Level Zero - There is no autonomy in a level zero vehicle. The driver is entirely in control of all functions related to operating the vehicle.
Level One - The vehicle performs some functions that might otherwise be left to the driver. Some kinds of driver technology, like automatic distancing between vehicles, qualify as level one autonomy.
Level Two - With level two autonomy, the vehicle takes over more functions. The vehicle controls steering and acceleration. However, the human operator must be there and ready to take over at any time.
Level Three - At level three automation, safety functions are handled by the vehicle. However, a human must still be present and ready to assume control. The car performs more sophisticated and discerning functions than a level two autonomous vehicle.
Level Four - All level four vehicles must be able to complete an entire trip from start to finish. However, level four vehicles don't perform in all scenarios. Under the right conditions, the vehicle can perform all of the functions of operating from start to finish.
Level Five - Level five is a fully automated vehicle. The vehicle has the same function and is equally as sophisticated as a human operator would be with a car in all situations. However, there is no involvement from a human operator at any time.