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which would connect East Carson Street with Second Avenue and an extension of Sixth Avenue.

This plan has the obvious advantage of starting some 40 feet higher than any of the present bridge approaches in the down town district, and at a point from 500 to 1000 feet north of any other feasible point of departure. Considerable gain is thus made at the very start. A uniform, uninterrupted gradient is proposed from Forbes Street to Washington Avenue, in order to climb the maximum amount possible with a given distance and gradient. Information furnished us through the office of Edwin K. Morse shows that the horizontal distance from Forbes Street to Washington Avenue is 6800 feet and the difference in elevation between the two points is 260 feet. It follows that a uniform gradient, from one end to the other would be 3.82 per cent; this could be reduced to 3.74 per cent by raising the grade of Forbes Street about 6 feet, a change which is to be desired in connection with the down town thoroughfare improvements and the Civic Center. At first sight this gradient seems good for Pittsburgh; but bearing in mind the distance -- over a mile and a quarter -- for which this gradient is maintained without a break, considerable hesitancy is felt about recommending it for the main artery of a large thoroughfare system. A gradient of 3.5 per cent should probably be considered a maximum for such a long climb on a main thoroughfare, and 3 per cent would be far better. The former gradient could be secured by dropping the southern end of the tunnel about 16 feet, the latter by dropping it about 50 feet. In either case, the southern end of the tunnel, or its approach, would be bent westward and extended a little down the valley, north of Washington Avenue, toward the mouth of the street car tunnel. The exact amount of reduction to be desired in the tunnel gradient must be finally determined in conjunction with a careful study of its southerly extensions based on complete and accurate surveys of the possible routes. There is no advantage in lengthening one portion of a thoroughfare to get a 3 per cent gradient if nothing better than 3.5 or 4 per cent can be secured on the rest of the route. The surveys made for the County Commissioners, under the direction of E. M. Bigelow, and courteously shown to representatives of the Civic Commission, unfortunately fail to furnish the needful data, as they do not extend to the upland

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Last modified on 22-Dec 1999
Design format: copyright 1997-1999 Bruce S. Cridlebaugh
Original document: Frederick Law Olmsted, 1910