|
As a rule, bridges for carrying both railway and highway traffic are located in or near large cities, although an occasional structure of this kind is found in country districts. The principal advantage of this type of bridge is the saving in first cost, and its principal disadvantage is a reluctance to cross over it on the part of timid drivers, whose horses may be frightened by the trains.
The saving in first cost of a combined railway and highway bridge as compared with two separate bridges for railway and highway traffic is considerable, because the piers for the combined bridge are but little, if any, more expensive than those for the railway bridge, and because the extra metal for the superstructure of the former in comparison with that of the latter is very much less in weight than the weight of metal required for a separate highway bridge.
The prejudice against combined bridges on account of danger is almost wholly unfounded, for horses soon become accustomed to railway trains, and, when screens are employed to hide the latter, but little trouble is experienced on account of frightened horses. These screens may be made either slatted or close, the former offering less resistance to the wind, and the latter being the cheaper.
The advent of the electric railway has somewhat complicated the question of designing combined bridges, for now it is often necessary to accommodate three or four kinds of traffic, viz., railway, electric, wagon, and pedestrian.
When a highway structure has to carry a single-track electric line in addition to the ordinary highway travel, the
133
|