The company cautions that production availability of highly-automated Level 4-5 operation depends on unpredictable 'external factors.'

BMW has outlined its roadmap for deploying autonomous technology in the coming years, shedding more light on significant challenges and unpredictable factors on the path to fully self-driving vehicles.

Breaking down the milestones using SAE International's five levels of automation, BMW has already achieved Level 2 advanced driver assistance that still requires "hands-on direction" from a human pilot at all times. The German automaker is confident it can achieve Level 3 automation with its iNext vehicle by 2021.

"During highly automated driving in traffic that is moving in the same direction and is segregated from oncoming traffic, the driver will be able to perform secondary in-vehicle activities for longer periods of time or relax (eyes off)," the company notes. "They must still be in a position to take over the task of driving again within a reasonable amount of time (a few seconds) when prompted by the system."

The iNext is expected to be technically capable of Level 4 autonomy by 2021, but such features may not immediately be available on production cars. The core improvement from Level 3 is a longer time span for taking over control ("mind-off"), allowing a driver to sleep during a long-distance journey, though the autonomous system might only work in certain pre-defined driving scenarios such as urban settings or divided highways.

Most automakers have painted a rosy picture of their autonomous ambitions. BMW appears to be breaking from the crowd, however, by disclosing a few caveats to help better educate drivers on the limitations of complex and evolving autonomous systems that will bridge the gap between current lane-holding tech and a Level 5 truly "self-driving" car that will not need a steering wheel.

"Current systems already work very well, yet they are certainly not yet able to substitute human intelligence in certain situations," the company says. "And the driver needs to be aware of this."

BMW will be relying on high-definition maps to extend a vehicle's reference data beyond the range of sensors. This can allow a vehicle to quickly recognize a system limitation or failure, providing more time for a human driver to take the wheel. For Level 4, the vehicle must be capable of navigating itself to a stop in a safe location if the driver is asleep or otherwise unable to immediately take control.

Transitioning to Level 5 operation will require another leap in capabilities, despite operating in the same manner as a Level 4 vehicle in most driving scenarios. A 'driver' will no longer be needed, and an unlicensed rider will never be obliged to take the wheel in any situation.

"This is exceedingly complex, so the demands placed on technical solutions are extremely high," BMW says. "To begin with, they will therefore be primarily deployed in city centers, where they will be used in delimited areas at first."

A fleet of 40 automated 7 Series prototypes will be on the road by the end of the year, paving the way for the iNext's arrival in 2021. The iNext is expected to be technically capable of Level 4-5 operation, but BMW cautions that deployment will depend on external factors that are currently difficult to predict.

Artificial intelligence is viewed as a key technology for safe self-driving cars. Automakers are still working to overcome significant challenges associated with AI data processing and programming, however, before the technology is ready to power Level 5 hardware platforms in production cars.

BMW suggests fully self-driving vehicles will conceivably arrive beteen 2020 and 2030.