https://phys.org/news/2018-05-deadliest-stage-self-driving.html

About a second before impact, the self-driving system determined that emergency braking was needed to avoid a collision. But, Uber had disabled the Volvo's factory-equipped automatic emergency braking system to avoid clashes with its own tech, the report said.

The NTSB also found that Uber's self-driving software had been trained not to apply its own emergency braking in situations that risked "erratic vehicle behaviour." This was done to provide a comfortable ride: Too many false positive detections (e.g. tree leaves, shrubs or plastic bags on the road) would result in a large number of emergency brakes which no passenger will tolerate.