How much do we really know about what so-called self-driving vehicles can and cannot do? The fatal traffic accident involving a Tesla Motors car that crashed while using its Autopilot feature offers a stark reminder that such drivers are in uncharted territory—and of the steep cost of that uncertainty.

The sensor systems that enable Tesla’s hands-free driving are the result of decades of advances in computer vision and machine learning. Yet the failure of Autopilot—built into 70,000 Tesla vehicles worldwide since October 2014—to help avoid the May 7 collision that killed the car’s sole occupant demonstrates how far the technology has to go before fully autonomous vehicles can truly arrive.