TOKYO -- A self-driving bus collision with a roadside tree in the Tokyo suburb of Hachioji in August was caused by the vehicle incorrectly reading its position just before the crash, authorities revealed on Nov. 10.

The bus hit the tree at around 11:30 a.m. on Aug. 29 during an experimental demonstration on National Route 20 in Hachioji's Takao area, inflicting minor injuries on three of the 15 people aboard. According to the Tokyo Metropolitan Bureau of Urban Development, the positioning error stemmed from a design flaw, and the bus's system for disabling sudden direction changes was also inactive.

According to the bureau, self-driving vehicles maneuver using positional information based on coordinates and vehicle direction which they read every 0.1 seconds. However, the vehicle collided with the tree on the left side of the road after it mistakenly read data from 22 seconds prior, causing it to turn suddenly to return to that position.

The onboard system to disable sudden turns does not activate unless the vehicle is traveling at around 30 kilometers per hour or faster, and the bus was moving at 20 to 25 kph at the time of the crash. While the tires were facing about 25 degrees to the left, the course instructions did not account for this, leading the bus to hit the tree. There was no independent collision avoidance system.

(Japanese original by Kazuo Yanagisawa, Tokyo City News Department)