582 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 



Further estimates indicated that a 1-line tunnel would have a 

 capacity about equal to the traffic demand at the opening of the 

 tunnel. A 2-line tunnel would have sufficient capacity to accommo- 

 date all traffic up to 1937, while a 3-line tunnel would reach its capac- 

 ity in 1943. 



Obviously it would be unwise to construct a 1-line tunnel whose 

 capacity would be reached as soon as put in operation. As between 

 a 2-line and a 3-line tunnel, it was found that the difference in cost, 

 with interest, would be sufficient to pay for another 2-line tunnel 

 after the first 2-line tunnel had outgrown its capacity. Of greater 

 importance was the consideration that no street or section could 

 accommodate the volume of traffic represented by a 3-line tunnel. 



If a 3-line tunnel were built, it could be operated at only 2-line 

 capacity. This would violate two of the main principles governing 

 proper tunnel planning — the distribution of traffic so as to avoid 

 undue congestion, and the investment of capital for construction only 

 as facilities are needed, without the necessity of providing for the 

 distant future. These are two of the most important features in 

 which tunnel construction is held to be superior to bridge construc- 

 tion in crossing wide, navigable rivers. 



The cost of a long-span bridge does not vary directly with the 

 span but increases about as the square of the span. On such a bridge 

 no conmiensurate saving in the cost of construction is obtained by 

 omitting some of its facilities. The tendency in bridge construction, 

 therefore, is to provide facilities greatly in excess of immediate 

 requirements, with a consequent expenditure of capital long before 

 those facilities are needed. Then when there is sufficient traffic to 

 utilize the bridge to full capacity, the resulting congestion in the 

 vicinity of the bridge entrances becomes a serious matter. This is 

 seen in the case of the East River bridges in New York City to-day. 



Tunnel construction, on the other hand, is more flexible than 

 bridge construction, because the cost is a direct function of its length, 

 with the volume of excavation increasing as the square of the 

 diameter. Since the cost of excavation represents a large part of 

 the total cost of a tunnel, any increase in the width of roadway can 

 be made only at considerable expense. The proper way to plan a 

 tunnel is to avoid the disadvantages inherent in bridge construction, 

 build only for the present and near future, and construct other 

 tunnels at other locations when the facilities of the first tunnel are 

 outgrown. 



Since a 2-line tunnel would have sufficient capacity to accommo- 

 date traffic up to 1937, and a 3-line tunnel would create such traffic 

 congestion in the vicinity of its entrances and exits as to preclude its 

 use to capacity ; also since the difference in cost between a 2-line and 

 a 3-line tunnel, with interest, would pay for a new 2-line tunnel 



