Panther Hollow bridge—a good-looking viaduct in Pittsburgh

There are certain general tendencies which are observable, both in America and in Europe, in cities which have a large area of hilltop land separated by deep valleys. The hills are generally preferred for residential purposes, and the earliest roads or trails often follow the ridges, plunging down and climbing up again steeply to get from one ridge to another. The main roads in the second stage of development are apt to seek the valleys for the sake of good gradients, with a corresponding development of the most active urban growth in the valleys and on the lower slopes; the hilltop development being retarded by lack of transportation facilities. Nevertheless the continued attractiveness of the uplands slowly builds them up, and as the wealth of the community grows there is an inevitable tendency to reduce the obstacles to ready connection between one hill district and another by raising the levels of the bridges which cross the intervening valleys. Bolder and bolder viaducts are built, until finally there is a complete and more or less independent highway system on the upper levels, and the major part of the residential district comes to be there too.

Obviously, therefore, every opportunity should be utilized to gain grade, in the approach to the South Hills District, by starting at a high elevation and wasting no distance in level stretches, if the most efficient thoroughfare artery to this district is to be secured.

A viaduct in Lausanne, showing how the valleys are spanned by the main traffic ways

The bridge and tunnel plan, proposed by residents of the South Hills, is briefly as follows: to start from Forbes Street, at Sixth Avenue, and rise steadily to the bluff north of Second Avenue; from here to rise on a bridge over the river, at a uniform gradient, to the opposite hill; to pierce the hill by a tunnel, at the same gradient, and reach the level of the present highways at the junction of Washington Avenue and Haberman Street. It is proposed also to have a lower deck on the bridge, 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 districts which are the ultimate objective of the proposed route.

[ROUGH PROFILES OF SOUTH HILLS TUNNEL AND THOROUGHFARE ROUTES]