As we exited the parking lot, my husband slowed the car to wait for a young woman riding her bike. When Tom nodded to indicate she could go first, she smiled, waved, and rode on. Moments later, the driver from a parked SUV threw his door open, knocking the young bicyclist to the pavement. Her legs bloodied, she cried as she examined her bent-up bike.

Later, we reflected on the accident: If only we had made her wait . . . If only the driver had looked before opening his door. If only . . . Difficulties catch us up in…