Though no one was injured or killed in the crash, which happened just past noon on Nov. 8, 2017, the NTSB undertook the investigation because the advent of autonomous vehicles is bound to result in crashes between driverless vehicles and traditional ones.
“The NTSB would normally not investigate a minor collision, but the involvement of a highly automated vehicle warranted having our investigators examine the circumstances surrounding the collision,” Kris Poland, deputy director of the NTSB’s Office of Highway Safety, said in a statement released with the report. “We wanted to examine the process of introducing an autonomous shuttle onto public roads as well as the role of the operator, the vehicle manufacturer, and the city."
The autonomous bus was operating as a shuttle on a 0.6-mile designated loop beginning and ending at a downtown shopping center. The battery-powered bus did not have a steering wheel, brake pedal, or accelerator pedal. Instead, it was operated by a handheld controller that looked much like those used to play video games. An attendant was aboard the bus.
The crash took place after the shuttle turned from Carson Avenue onto South 6th Street. A tractor-trailer truck was attempting to back into an alley off 6th street. The following is what the NTSB release said happened next.
" The truck driver said he saw the shuttle turn onto 6th Street and assumed it would stop a ‘reasonable’ distance from the truck . . . The shuttle’s sensor system detected the truck at 147.6 feet and tracked it continuously as it backed up. Programmed to stop 9.8 feet from any obstacle, the shuttle began to decelerate when it was 98.4 feet from the truck. When the shuttle was 10.2 feet from the truck and nearly stopped, the attendant pressed one of the emergency stop buttons. The attendant and passengers waved to gain the truck driver’s attention but 11 seconds after the shuttle stopped, the right front tire of the slow-moving truck struck the shuttle"
Based on its investigation, the NTSB concluded that the probable cause of the crash “was the truck driver’s action of backing into an alley, and his expectation that the shuttle would stop at a sufficient distance from his vehicle to allow him to complete his backup maneuver.”
A contributing factor, the NTSB said, was that the bus attendant was not in a position to take manual control over the bus.