In order to make the investigation general instead of particular, so as to apply to all other similar river crossings by very-high-level suspension- bridges, the span-lengths were assumed to be 1,500, 2,300, and 3,000 feet, to permit the plotting of curves of total costs for both bridges and tunnels with their approaches for various widths of river. In this special case the span length would have to be about 2,900 feet; and in all cases the length

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of the horizontal portion of the tunnel has been assumed to be exactly equal to that of the main span of the competing bridge.

In the highway-structure comparison there were adopted for the bridge three clear roadways of twenty-two feet each, and two sidewalks of eleven feet each, corresponding to four double-track tubes each of twenty-two feet

clear roadway. The driveways were assumed to consist of creosoted-