The key points of interest from the preliminary report, emphasis and (comments) mine  report source

The vehicle was factory equipped with several advanced driver assistance functions by Volvo Cars, the original manufacturer. The systems included a collision avoidance function with automatic emergency braking, known as City Safety, as well as functions for detecting driver alertness and road sign information. All these Volvo functions are disabled when the test vehicle is operated in computer control but are operational when the vehicle is operated in manual control.

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According to Uber, the developmental self-driving system relies on an attentive operator to intervene if the system fails to perform appropriately during testing. In addition, the operator is responsible for monitoring diagnostic messages that appear on an interface in the center stack of the vehicle dash and tagging events of interest for subsequent review.

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

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In a postcrash interview with NTSB investigators, the vehicle operator
stated that she had been monitoring the self-driving system interface. The operator further stated that although her personal and business phones were in the vehicle, neither was in use until after the crash, when she called 911.

(no information on whether the NTSB investigators requested subpoena of operator/driver phone records available at this time)

(The rumored Arizona Tempe Police Department’s 318 page accident report states that Vasquez’s Hulu account had accessed the Voice TV show for 42 minutes before the crash, and had continued access till accident occurred.)

N.B. Arizona, one of the few US states, does not halve laws against use of cellphones while operating vehicles on public roads and highways.