The next time you’re traveling on the interstate and see a fellow driver whose hands are full with everything but the wheel—scrolling TikTok, applying mascara, eating breakfast—don’t panic. It’s all legal in certain states, as long as they’re in a new Mercedes with autonomous driving technology. The luxury automaker has become the first in the nation to start selling self-driving cars—at least those that afford riders a hands-free experience—to regular consumers. As of April 11, there were 65 Me...