… is from Book V, Chapter 1 – on page 725, Vol. II, of the 1981 Liberty Fund edition – of Adam Smith’s 1776 An Inquiry Into the Nature and Causes of the Wealth of Nations:
When high roads, bridges, canals, &c. are in this manner made and supported by the commerce which is carried on by means of them, they can be made only where that commerce requires them, and consequently where it is proper to make them. Their expences too, their grandeur and magnificence, must be suited to what that commerce can afford to pay. They must be made consequently as it is proper to make them. A magnificent high road cannot be made through a desert country where there is little or no commerce, or merely because it happens to lead to the country villa of the intendant of the province, or to that of some great lord to whom the intendant finds it convenient to make his court. A great bridge cannot be thrown over a river at a place where nobody passes, or merely to embellish the view from the windows of a neighbouring palace: things which sometimes happen in countries where works of this kind are carried on by any other revenue than that which they themselves are capable of affording.