Why are the young trees described as 'sprinting'?

While driving to the airport, in an effort to distract herself from the thoughts of her ageing mother, the poet looks at the young trees ‘sprinting’. The trees seem to be running past the moving car. The sprinting of the trees symbolises the rapidly passing years of human’s life from childhood to old age. This image, which shows activity and strength, is contrasted with that of her old and weak mother who seems dormant, sleeping in the car. The ‘young’ trees represent life in contrast to her mother’s approaching death.