In all, Brown had his hands off the wheel for 90 percent of his final drive, according to Tesla vehicle data reviewed by the National Transportation Safety Board. The NTSB on Monday released a roughly 500-page report detailing the facts of the case, but declined to provide a final analysis or judgment because the investigation is ongoing.
Included in the evidence are details about the highway where the crash occurred, both vehicles involved and, crucially, behavioral information from the Tesla that sheds light on Brown's activities immediately before the accident. The case is being closely watched, because the outcome of the investigation could affect consumer attitudes toward Tesla, automation and self-driving technology in particular.
Brown's final drive in his Tesla lasted 41 minutes, according to the NTSB. Of those 41 minutes, 37 were spent with the autopilot enabled. Autopilot is Tesla's term for its high-end cruise control feature that can help a vehicle stay in its lane semi-autonomously. The company declined to comment on the report.
Tesla requires its drivers to keep their hands on the wheel even when Autopilot is engaged. But Brown appears to have ignored those warnings, even as he manually increased the autopilot's speed 2 minutes before he crashed into the truck, according to the NTSB report.
Earlier reports by NTSB on the crash have concluded that in addition to going hands-free for the majority of the trip, Brown also made no effort to brake, steer or otherwise avert the deadly accident.
Since the crash, those reports have said, Tesla has updated its Autopilot feature to include a strikeout system, whereby drivers who repeatedly ignore safety warnings risk having their Autopilot disabled until the next time they start the car.
At the time of the incident, Tesla said that it was the first crash involving Autopilot in roughly 130 million miles of driving in which the technology was in control. The United States suffers a death on the roads about once every 100 million vehicle miles traveled, according to the Insurance Institute for Highway Safety.
But while autonomous or semiautonomous driving technology could help reduce collisions in general, questions about how and when to draw the line between manual and autonomous mode have yet to be fully resolved by engineers and researchers. Last year, for example, a study by Stanford University found that drivers often had trouble taking the wheel again after letting a computer drive, even momentarily. Drivers commonly over- or undercorrected with the steering wheel, even when they knew the handoff was coming, the research found. The effects were more pronounced if driving conditions had changed substantially since the last time the drivers were in control, according to a Stanford release.