Someday soon, cars may drive themselves, and perhaps we will be better off for it. Until then, driving remains a human task, subject to fundamental limits on our ability to pay attention. The National Safety Council estimates that in 2013 alone, 1.1 million crashes involved using a phone, and the Transportation Department counted more than 3,000 deaths and 400,000 injuries caused by distracted driving that same year.

By now everyone knows about this problem. Adding a simple feature called “Driving Mode” to all mobile phone...