Elon Musk’s push to extend Tesla’s driving assistance to cities ranks a leading safety authority

Tesla Inc. is preparing a major upgrade to its driver assistance software. The country’s top accident investigator said the move may have been premature.

Chief Executive Elon Musk said last week that drivers may soon be requesting an upgraded version of what Tesla calls its “full autonomous driving capability.” The upgrade is expected to add functionality to help vehicles navigate cities, expanding the suite of driver assistance tools that had been designed primarily for highways.