While Tesla is clear that drivers of its electric vehicles can take their hands off the wheel for extended periods when Autopilot is engaged, their monitoring of the road conditions are still required.
The NTSB says Brown ignored repeated warnings from Autopilot to retake control of the vehicle.
The Tesla Model S accident that past year sparked a debate about the auto company's partially autonomous driving tools resulted in a cloud of white dust as the sedan ploughed underneath a tractor trailer, a witness told investigators. It contains hundreds of pages of crash reconstruction report, description of the crash sequence, interview transcripts and summaries, photographs, and other investigative material.
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The NTSB's report says that two Autopilot systems were active - adaptive cruise control, which allows the vehicle to set and maintain a pace relative to traffic in front of the auto, and Autosteer, which holds the vehicle in its lane.
The NHTSA earlier this year found that the owner of the Tesla Model S sedan had ignored the manufacturer's warnings to maintain control even while using the driver-assist function.
After looking through the electronic devices in the vehicle, NTSB found no evidence that they were in use and its witness accounts don't match what was previously reported.
NHTSA said Brown did not apply the brakes and his last action was to set the cruise control at 74 miles (119 km) per hour less than two minutes before the crash - above the 65 mph speed limit.The agency said the truck should have been visible to Brown for at least seven seconds before impact. Brown did not manually apply the brakes either with his last input to the auto being setting the cruise control to 74 mile per hour in a 65 zone.
The driver of the truck had had his license suspended five times between 1984 and 2013 for violations including speeding and failure to appear in court, according to NTSB. Analysis, findings, recommendations, and probable cause determinations related to the crash will be issued by the Board at a later date.