Getting out of my car along the sparsely driven yet frenetic, four-lane Memorial Drive shortly after Monday’s fatal accident involving a transit bus and 11-year-old boy on a bicycle, I veered on the sidewalk past a woman and three children—as they walked peacefully toward the unknowing calamity ahead—and saw through scattered collections of onlookers standing and watching, some at sidewalk level and others on slopes overlooking the scene, in stunned silence.