other selected streets may be carried over or under the express highway, without access to it. All other streets should be terminated at the parallel local service ways which, in cities, will always be required.
Various means of reducing the number of interrupted streets and grade separations (by suitable location of the express routes) have been discussed in a previous section of this report.
To avoid undue obstruction of the cross movement of pedestrians, foot bridges should be constructed to span the express ways at frequent intervals.
Generally in the largest cities, and under some circumstances in smaller cities, a satisfactory meeting of the conditions imposed, especially near the city center, may require the raising or lowering of extended sections of the interregional route above or below the adjoining ground level, in order to carry it over or under frequent cross streets or over some and under others. Where the general topography of the city in such sections approaches a level or uniformly sloping plane, continuous elevation or depression of the express route is the indicated solution. Where the topography is rolling, the most feasible grade profile may be one cutting through the natural roll and thus passing over some cross streets and under others.
The effort to crowd an elevated highway into the narrow space generally afforded by existing surface streets will usually result in unsatisfactory design of the express route and impairment of the utility of the surface street for local service. Generally, it will also cause serious damage to abutting property. To avoid these undesirable consequences it will usually be necessary to acquire a right-of-way wider than can be found within the limits of an existing street. This may be done by taking the added width at one side of a street; or a more feasible location, avoiding the taking of property frontage, may be found at the rear of properties fronting on adjacent streets. By location of the latter type, damage to adjoining property may, under some conditions, be lessened. In general, the Committee considers elevation of the express routes a solution acceptable only in a commercial or business environment, as shown in plate VIII. It shares what it believes to be a widely held opinion opposing the cutting of such facilities through residential areas.
Depression of the express route will usually require extensive reconstruction of underground facilities, such as water mains, sewers, and electric conduits; and at low elevations drainage may be difficult and expensive. It will rarely be possible to achieve full depression within the width of an existing street. Additional right-of-way acquisition will nearly always be involved. The razing of numerous existing buildings will usually be necessary also; but this under many circumstances, particularly in blighted areas, may be regarded as an end desirable in itself.
Such are the principal difficulties of depressed construction. Where they can be overcome, the resulting development may be considered by many, more pleasing to the eye and more consonant with a gracious improvement of the urban environment than any other solution of the express-highway problem. Wholly satisfactory design will usually require condemnation of a block-wide strip of property through the city, retaining the existing surface streets at the two ends of the block as local service ways.
