The top US auto-safety regulator has opened an investigation into Zoox, the autonomous-vehicle subsidiary of Amazon.com Inc., after two incidents in which its SUVs suddenly braked and were rear-ended.The US National Highway Traffic Safety Administration’s Office of Defect Investigation opened a preliminary evaluation after receiving notice of two Toyota Highlander sport utility vehicles equipped with Zoox’s automated-driving system braked unexpectedly. In both cases, motorcyclists collided with ...