Why is it important to leave extra space behind a large vehicle on a hill?

• The large vehicle may roll in front when the driver releases the brakes.
• The large vehicle may roll back when the driver releases the brakes.
  Explanation:

  "If you are on a hill and stopping behind a large vehicle, leave extra space in front of your vehicle. The large vehicle may roll back when the driver releases the brakes."
• The large vehicle might not stop at the red light.
• The large vehicle does not need to stop at a red light.