Doing it in half the time would be impossible. To travel 250 miles in three hours you'd need to keep up an average of 83mph-ish for the whole journey. I should

Doing it in half the time would be impossible. To travel 250 miles in three hours you'd need to keep up an average of 83mph-ish for the whole journey. I shouldn't think there are many places you can reach a peak speed of 83mph for long on the whole road.