For years, Tesla CEO Elon Musk has been making bold claims that the “Autopilot” feature that allows Tesla drivers to drive hands-free is safer than traditional driving.

In 2016, for instance, he said that “half a million” people would be saved if Autopilot were more widely available. In 2017, Musk tweeted that the latest Autopilot software update could reduce collisions by “90%.” And when a government agency found that equipping cars with Autopilot decreased the rate of crashes by 40%, Mr. Musk spread it enthusiastically.