Accidents in which children die after being left in vehicles are more common than one might think. According to car safety expert Kimberly Martin, more than 40 children died in hot cars in the United States last year alone. She said advocacy group KidsAndCars.org has reported 17 such deaths so far this year, and awareness of the problem has increased after the highly publicized death of 22-month-old Cooper Harris in Cobb County last month.