Tesla's Autpilot requires drivers to keep their hands on the wheel (Photo: Tesla)

Tesla said a deadly crash involving a Model S doesn't cast doubt on the viability of its autonomous driving technology.

The Californian electric car maker was responding to a preliminary investigation by US authorities into an accident that occurred recently when a Model S with its Autopilot system switched on hit a tractor trailer, killing one person.

Autopilot is the name of Tesla's autonomous-drive function, which allows its cars to independently manage speed, stay in lane, change lanes and park. It is switched off by default, but can be turned on easily by a driver.

Tesla pointed out that this was the first Autopilot-related fatality in more than 130 million miles driven, compared with one fatality every 94 million miles driven for all vehicles and a comparable worldwide rate of one for every 60 million miles.

Moreover, the carmaker said, its Autopilot technology is still in a public beta phase, requiring drivers to keep their hands on the steering wheel at all times. But, as more driving miles are accumulated, Tesla predicted that "the probability of injury will keep decreasing."  Already today, the company added, "when used in conjunction with driver oversight, the data is unequivocal that Autopilot reduces driver workload and results in a statistically significant improvement in safety when compared to purely manual driving."

Explaining how the accident happened, Tesla said in a blog post that the Model S was driving on a divided highway with Autopilot engaged when a tractor trailer drove across the highway. Neither Autopilot nor the driver noticed the white side of the tractor trailer against a brightly lit sky, Tesla said. Hence, the brake was not applied.

"The high ride height of the trailer combined with its positioning across the road and the extremely rare circumstances of the impact caused the Model S to pass under the trailer, with the bottom of the trailer impacting the windshield of the Model S," Tesla said. "Had the Model S impacted the front or rear of the trailer, even at high speed, its advanced crash safety system would likely have prevented serious injury as it has in numerous other similar incidents."