The ad shows: a car cruising on its own. The manual warns: driver, never stop paying attention

An Audi A8 in testing mode works on refining a suite of semi-autonomous driver-assist features that allow drivers to hand over some tasks to the automobile, though they must check in with the steering wheel often to keep the system active. Car companies battling for their share of $2 trillion in annual global auto sales increasingly lean on shiny tech that takes over some of the driving from humans. Boasting names such as Autopilot, Super Cruise and ProPilot Assist, these systems -- whose radar and cameras are the building blocks of self-driving cars -- are part of a growing effort by manufacturers to woo with computing power rather than horsepower. But on the heels of two Teslas that crashed while on Autopilot, automakers find themselves increasingly torn between hyping the tech and warning owners about its limitations. Nissan's ProPILOT Assist technology, which is paving the way for future fully autonomous vehicles, is aimed at reducing the hassle of stop-and-go driving by helping control acceleration, braking and steering during single-lane highway driving. But drivers must constantly oversee the system, Nissan says.