been shown, but probably not all of them; and as for the economics, it has been proved to cost less, even in extreme cases, than a very-high-level structure, and about the same as a low-level structure; but that, strictly speaking, it cannot be said to be in economic competition with either type; because when the conditions really call for the consideration of a transbordeur, a very-high-level bridge would generally necessitate too great a climb for the traffic, and a low-level bridge would be barred because of its interfering too much with the paramount interest of navigation. The Philadelphia-Camden case is apparently an exception to this rule, but it must be remembered that its vertical clearance is only 135 feet, while that for the proposed crossing at New Orleans is 175 feet, and that the extra forty feet of height make a great difference in the costs of the approaches.