which covered only the 37,700 miles designated in 1947.
Wikipedia says that the Interstate Highway System has a total length of 46,876 miles as of 2006.
So that's quite a bit more than what they had planned.
Yes, agreed it is quite a bit more, about 30% more miles then planed, yet the cost went up by a factor of over 400%. Remember, the estimate was for 25 Billion. If we allowed for a doubling of the miles, the cost should have been 50 Billion, but it was over 100 Billion! I do hope you are not trying to tell me that government cost estimates are a realistic representation of what will actually be spent.