GOTEBORG, Sweden — One of the big unknowns about self-driving cars is how human drivers will react once they can hand off the tasks of steering, braking and avoiding obstacles to a combination of unseen sensors, software and computer chips under the hood.

Volvo Cars is trying to find an answer. It is putting video cameras into cars equipped with its latest driver-assistance features and giving them to five families to record their actions while rolling along in highway traffic.

The aim is to gather data that will help Volvo develop fully autonomous cars for highway use by 2021, Marcus Rothoff, a Volvo executive heading the project, said at an event here Monday. By then, the person in the driver’s seat will be able to watch a movie while the car navigates the morning commute, he said — but only if people are comfortable letting go of control.

“If you don’t trust the car, you can’t do other things,” he said. “We need to make a car that people trust and have confidence in.”