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Rh highways and connecting city streets may suffice and may be feasible; although on the rural highway sections involved, widening should always include a physical separation of the opposing traffic streams, and any widening in the downtown areas of cities is certain to be beset with difficulties.

In the larger cities generally only a major operation will suffice—nothing less than the creation of a depressed or an elevated artery (the former usually to be preferred) that will convey the massed movement pressing into, and through, the heart of the city, under or over the local cross streets without interruption by their conflicting traffic. Such facilities are not required in any city for the service of through highway traffic alone. They are not required solely for the service of the traffic entering the city from typically rural highways. There usually is added to these streams in the outer reaches of the city or its immediate suburbs a heavy movement of purely city traffic that mounts to high peaks in the morning and evening rush hours. Movements of this latter sort largely follow the same lines as the traffic entering the city from main rural highways simply because the peripheral city areas and suburbs in which they are generated have developed along such highways. There are cases in which the daily peak of ‘‘in-and-out” city traffic exists without any substantial addition from main rural highways. For such cases the requisite facility—an express highway—is in all essentials similar to facilities designed to carry external traffic across the city.

Whether the needed facility be a transcity connection or an express highway, or whether the traffic to be served includes large or insignificant contributions from extra-city highways, in either case the nature of the traffic within the city is much the same.

It always is largely a movement from the periphery to the center of the city, and is little concerned with intermediate city sections, but it must pass through them and, in so doing, is obstructed more or less frequently at the cross streets. The congestion that results, under present conditions, is due in part to the usually inadequate width of the existing artery and in part to conflict with cross traffic, generally complicated by parked vehicles.

It has been remarked previously that back of the failure to enlarge the capacity of a main rural highway at the approach to the city there are basically right-of-way difficulties. If this is true in the environs of the city, it is most emphatically true with respect to such needed improvements within the city as have been described in the foregoing paragraphs. Because these difficulties seem to the municipal administration virtually insurmountable such major improvements have thus far been attempted in very few instances.

Outstanding among the few instances that can be cited, both for their completeness and the vigor of their execution are the West Side Highway and Henry Hudson Parkway in New York City, together with their connecting parkways in Westchester County, N. Y., and the Merritt Parkway in Connecticut. Less complete but still admirable in its conception and bold in execution is the short section of depressed highway recently constructed in St. Louis; and remarkable as an earlier, less daring venture, that can be converted with relative ease into a highly efficient modern facility—the Roosevelt Boulevard in Philadelphia. In its present form the latter consists of a central artery for through traffic, bordered at each side by local service lanes,