Thirty years ago, Louie Psihoyos watched as two young children were killed a few steps in front of him. On a blue-sky day near a market in Perkiomen, Pennsylvania, he noticed a family walking hand-in-hand beside the road ahead. A large van swept past and he saw that one of its giant wing mirrors would hit one of the children as it tore past. Psihoyos shouted, but it was too little too late. Both children were dragged beneath the van's wheels and crushed. "They died right in front of me," says the 58-year-old, Oscar-winning filmmaker, his moss-green eyes stretching in...