Before last month, Google’s autonomous cars had driven 1.4 million miles over six years without causing a single accident. Then three weeks ago, in Google’s hometown of Mountain View, Calif., an old-school public bus made a sudden move not predicted by Google’s algorithm. The Google car—going two miles per hour—hit the bus, which not surprisingly was traveling faster.

No one was hurt, but Google, a unit of Alphabet (ticker: GOOGL), took the accident far more seriously than your typical fender bender—as if the future of…