Trumpeted as a defining moment on the road to full autonomy, the system was the first in production that allowed humans to hand driving responsibility to the car itself for some portion of the journey — so long as a human remained available as a backup. In industry jargon, such a system is classified as Level 3 automation.

But the milestone arrived with an asterisk.

Audi equipped the A8 with all the components necessary to make Traffic Jam Pilot work, but it hadn't actually enabled the feature. Activation would come, brand executives theorized at the time, via over-the-air updates as regulatory compliance was ensured market by market.

Today, Traffic Jam Pilot remains dormant in the U.S. Audi has no foreseeable plans to activate the system, and the future of Level 3 automation for Audi and everyone else remains beset by a morass of regulatory, technical, safety, behavioral, legal and business-related complications.

Once, Level 3, sometimes called "conditional automation," seemed like a natural step in the evolution of automated technology, a progression beyond today's driver-assist systems in which drivers retain responsibility for vehicle operations and a precursor to self-driving vehicles. But that building-block assumption baked into the Levels of Driving Automation, outlined by SAE International, belied unique challenges.

"Some OEMs say they're going to skip Level 3 altogether and do Level 4, while others are still pursuing this possibility," says Christophe Marnat, executive vice president of the electronics and advanced driver-assist systems division at ZF. "I'm not sure how to interpret that. Does that mean they have solutions? … Or is it just research that never comes to fruition? It's questionable."