Put it this way: If you were to average 60 mph over 2 miles, you would travel those two miles in 2 minutes. However, if you've already driven the first mile a

Put it this way: If you were to average 60 mph over 2 miles, you would travel those two miles in 2 minutes. However, if you've already driven the first mile at 30 mph, it's taken you two minutes to drive it. In order to then average 60 mph over the full two miles, you'd have to travel the remaining mile in 0 seconds -- in zero time.