Passage: Set in modern times, a young five-year-old boy named Johnny McGowan travels to Scotland to live at his grandfather's castle after he loses both his parents in a traffic collision. At the magical wish tree on his grandfather's estate, he conjures up a friend which is an infant dragon whom he nicknames "Yowler". They grow up together as 15 years go by. One day after the years go by, documentary filmmaker Bob Armstrong, his daughter Beth, and his pilot Brownie McGee stumble upon Yowler. Eager for fame and money, Bob convinces John to "rent" Yowler to local corrupt businessperson, Lester McIntyre. John, who is coerced in part by the offer to have the mounting taxes on the castle paid off, allows Lester to take Yowler in. He does so also partly because of his growing interest in Beth. Yowler is miserable and harassed in the new amusement park built for him, and when it becomes clear that McIntyre has duped them in order to exploit the dragon, John and his new friends take action.

Question: What was the cause of death for this young boy's parents?
traffic collision