DETROIT -- Soon we'll be driving vehicles that can steer, accelerate and brake themselves on the highway without the motorist's intervention.

The technology isn't foolproof, however, so drivers must be ready to take over if the vehicle encounters an unexpected obstacle. But if a motorist is texting a spouse or -- heaven forbid -- taking a nap, he or she may be too distracted to do so.

Indeed, if autonomous driving is Megatrend No. 1 in the global industry, the matter of how to handle the human interface may be Conundrum No. 1. It was certainly a hot topic last week here as auto executives debated the technology of driverless cars during the annual SAE World Congress, an event that attracted thousands of engineers.

"This is one of the most challenging parts of the development," said Cadillac spokesman David Caldwell. "There are many schools of thought and many concepts being tried."

Automakers have devised different techniques to ensure that the motorist stays alert. Some, such as BMW, require drivers to keep their hands on the steering wheel. Others, such as General Motors, plan to use other techniques.

The stakes are huge. It might take just a few well-publicized accidents to convince motorists that self-driving cars can't be trusted. If so, billions of r&d dollars would go up in smoke.