At 2:44 p.m. Tuesday, July 29, every cellphone within reach of Hawaii’s Emergency Management Agency blared with a warning sound designed to raise listeners’ adrenaline, and a message appeared, warning, “Take Action! Destructive tsunami waves expected.” The alert directed all to “EVACUATE coastal areas in PRIMARY EVACUATION ZONE,” and linked to a map of the most endangered areas.

Once the first wave reached Honolulu, shortly past 7 p.m., it appeared likely that Hawaii would escape major damage. That bore out. The state’s emergency alerts did their job well, in so far as making people aware that a tsunami threatened. But confusion over evacuation zones was common, and paralyzing traffic jams could have led to far worse outcomes had a destructive tsunami arrived.

Honolulu’s experience can n

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