Tesla Motors Inc. has been cleared by the U.S. National Highway Traffic Safety Administration in its investigation into the fatal car crash of a Tesla driver who was using the company’s autopilot feature.

Joshua Brown died in May behind the wheel of his 2015 Model S when neither Tesla’s Autopilot nor Brown himself noticed a tractor-trailer driving perpendicular to the vehicle crossing its path due to light conditions. The ride height of the trailer combined with its position on the road ultimately caused the Model S to pass under the trailer, with the bottom of the trailer hitting the windshield and ultimately killing the 40-year-old driver.

The NHTSA’s Office of Defects launched an investigation into the fatal accident in June to examine the design and performance of the automated driving systems in use at the time of the crash, in order to decide whether a defect in Tesla’s Autopilot led to the accident.

In dismissing any liability held by Tesla, the investigation report found that driver error was to blame. Brown would have seen the tractor trailer for at least seven seconds prior to impact, the report said, but given that he was not paying attention to the road he did not act. It was alleged that at the time of the crash, Brown was watching a Harry Potter movie.

The findings did, however, say that makers of autonomous driving systems have an obligation to make sure drivers were paying attention. “An attentive driver has superior situational awareness in most of these types of events, particularly when coupled with the ability of an experienced driver to anticipate the actions of other drivers,” the report noted. “Tesla has changed its driver monitoring strategy to promote driver attention to the driving environment.”

According to The Guardian, U.S. Transportation Secretary Anthony Foxx told reporters on Thursday that drivers have a duty to take seriously their obligation to maintain control of a vehicle. He said automakers also must explain the limits of semi-autonomous systems, such as in this case where Tesla’s Autopilot failed to detect the tractor-trailer.

“The (auto) industry is going to have to be clear about what the technology does and what it is does not do, and communicate it clearly,” Foxx said.

Report highlight: “The data show that the Tesla vehicles crash rate dropped by almost 40 percent after Autosteer installation.”

— Elon Musk (@elonmusk) January 19, 2017

The findings of the investigation went further than simply exonerating Tesla, as NHTSA noted that vehicles equipped with Tesla’s Autopilot were 40 percent less likely to be involved in a fatal accident.

Preempting the report, Tesla had already rolled out a new version of its Autopilot software that, in addition to delivering improved vehicle detection, added extra emphasis on driver attention such as a requirement for drivers to touch the steering wheel more frequently.

Image credit: chijs/Flickr/CC by 2.0