Uber had disabled emergency braking in a self-driving test vehicle that struck and killed a woman in Tempe, Arizona, last March, a preliminary report from the National Transportation Safety Board (NTSB) said.

The modified 2017 Volvo XC90 was occupied by one vehicle operator and operating with a self-driving system in computer control mode, the NTSB report said.

The report states data obtained from the self-driving system shows the system first registered radar and LIDAR observations of the pedestrian about six seconds before impact, when the vehicle was travelling at 43 mph. As the vehicle and pedestrian paths converged, the self-driving system software classified the pedestrian as an unknown object, as a vehicle, and then as a bicycle with varying expectations of future travel path. At 1.3 seconds before impact, the self-driving system determined that emergency braking was needed to mitigate a collision.

But, according to Uber, emergency braking maneouvres are not enabled while the vehicle is under computer control to reduce the potential for erratic vehicle behaviour. The vehicle operator is relied on to intervene and take action. The system is not designed to alert the operator.

In the report the NTSB said the self-driving system data showed the vehicle operator engaged the steering wheel less than a second before impact and began braking less than a second after impact. The vehicle operator said in an NTSB interview she had been monitoring the self-driving interface and that while her personal and business phones were in the vehicle neither were in use until after the crash.

The NTSB's preliminary report, which by its nature does not contain probable cause, states the pedestrian was dressed in dark clothing, did not look in the direction of the vehicle until just before impact, and crossed the road in a section not directly illuminated by lighting. The pedestrian was pushing a bicycle that did not have side reflectors and the front and rear reflectors, along with the forward headlamp, were perpendicular to the path of the oncoming vehicle. The pedestrian entered the roadway from a brick median, where signs facing towards the roadway warn pedestrians to use a crosswalk, which is located 360 feet north of the Mill Avenue crash site. The report also notes the pedestrian's post-accident toxicology test results were positive for methamphetamine and marijuana.

In its report the NTSB said Uber equipped the test vehicle with a developmental, self-driving system, consisting of forward- and side-facing cameras, radars, Light Detection and Ranging, navigation sensors and a computing and data storage unit integrated into the vehicle. The vehicle was factory equipped with several advanced driver assistance functions by the original manufacturer Volvo Cars, including a collision avoidance function with automatic emergency braking as well as functions for detecting driver alertness and road sign information. The Volvo functions are disabled only when the test vehicle is operated in computer control mode.

All aspects of the self-driving system were operating normally at the time of the crash, and there were no faults or diagnostic messages.

The preliminary report contains no analysis and does not discuss probable cause.

"The information in the report is preliminary and subject to change as the NTSB's ongoing investigation progresses. As such, no conclusions about probable cause should be drawn from the information in the preliminary report," the agency said.

Reuters said the report gave new fuel to opponents in Congress who have stalled a bill designed to speed the deployment of self-driving cars on US roads and puts a spotlight on the fact that the National Highway Traffic Safety Administration, which is also investigating, does not test self-driving vehicles or certify them before they are deployed on US roads.

Uber has said it aims to resume its self-driving operations this summer, likely with smaller routes and fewer cars, Reuters noted.

The company did not directly comment to the news agency on the NTSB findings but noted it recently named a former NTSB chairman, Christopher Hart, to advise on Uber's safety culture.

""As their investigation continues, we've initiated our own safety review of our self-driving vehicles programme," the company said, adding it planned to announce changes in the coming weeks.

According to Reuters, William Wallace, senior policy analyst for Consumers Union, the advocacy division of Consumer Reports, called Uber "reckless" and said the NTSB report "makes it clear that a self-driving car was tested on public roads when it wasn't safe enough to be there, and it killed a pedestrian". He added that the system "was far too dangerous to be tested off a closed track".

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