https://phys.org/news/2018-05-apnewsbreak-tesla-autopilot-sped-utah.html

Police suggested that the car was following another vehicle and dropped its speed to 55 mph to match the leading vehicle. They say the leading vehicle then likely changed lanes and the Tesla automatically sped up to its preset of 60 mph (97 kph) without noticing the stopped cars ahead of it.

The police report, which was obtained Thursday through an open records request, provides detail about the vehicle's actions immediately before the May 11 crash and the driver's familiarity with its system.