Even as automakers and technology companies have been promoting a euphoric vision of the future in which cars will drive themselves and serious crashes will be rare, their engineers have been engaged in a sobering debate.

Just how autonomous can and should cars become? the engineers are asking. Is there an inherent danger in technology that invites human drivers to sit back and relax — but still requires them to be ready to hit the brakes or grab the wheel at the first sign of trouble?

Those questions have taken on a new urgency after the revelation this week that the driver of a Tesla Model S died in a crash in Florida while the electric car was operating in its Autopilot mode.

The man, Joshua Brown, 40, of Canton, Ohio, was driving on a divided highway, when a tractor-trailer truck made a left turn and crossed in front of Mr. Brown’s lane of traffic. Tesla said neither Mr. Brown nor the car’s self-driving system noticed the white truck against a bright sky, and the brakes were never applied.