The chairman of the US National Transportation Safety Board (NTSB) said on Tuesday that "operational limitations" in the Tesla Model S played a "major role" in a May 2016 crash that killed a driver using the vehicle's semi-autonomous Autopilot system. The limits on the autonomous driving system include factors such as Tesla being unable to ensure driver attention even when the car is traveling at high speeds, ensuring Autopilot is used only on certain roads and monitoring driver engagement, NTSB said. The NTSB recommended auto safety regulators and automakers take steps to ensure that semi-autonomous systems are not misused. "System safeguards were lacking," NTSB chairman Robert Sumwalt said. "Tesla allowed the driver to use the system outside of the environment for which it was designed and the system gave far too much leeway to the driver to divert his attention."