Why was Dr. Johnson driving carefully?

Nearly at nine o’clock in the evening, suddenly the telephone was ringing. Dr. Johnson went to answer it. He was asked to go and operate on a very sick boy. The boy was in hospital He had shot himself while playing with a gun. His life was in danger. After he heard this, Dr. Johnson decided to go at once.
    It was snowing. Dr. Johnson was driving carefully. He thought he could get to the hospital before twelve o’clock though the hospital was sixty miles away from his city. A few minutes later, Dr. Johnson’s car had to stop for a red light at a pressing. Suddenly a man in the old black coat with a gun got in the car. He ordered the doctor to get out of the car, and then he drove on down the road. Dr. Johnson had to go by train, but got to the hospital late. The boy’s father, with his head in his hands, was the man in the old black coat.