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Rh The interregional routes, however they are located, will tend to be a powerful influence in shaping the city. For this reason they should be located so as to promote a desirable development or at least to support a natural development rather than to retard or to distort the evolution of the city. In favorable locations, the new facilities, which as a matter of course should be designed for long life, will become more and more useful as time passes; improperly located, they will become more and more of an encumbrance to the city’s functions and an all too durable reminder of planning that was bad.

It is very important, therefore, that the interregional routes within cities and their immediate environs shall be made part of the planned development of other city streets and the probable or planned development of the cities themselves. It is well to remember in this connection that observations of the existing traffic flow may not be an infallible guide to the best locations.

In many cities there are city planning commissions that have already given thought to desirable changes in the present city structure. Some of these bodies have reached quite definite decisions regarding many of the elements that will affect the location of interregional highways in and near the city. Usually the decisions of the planning commission have grown out of studies of the city as it is, and as the commission desires it to be. And these studies will usually afford the principal data and bases for agreement upon the general locations of the interregional routes.

It is especially desirable that the agreement have the full concurrence of housing and airport authorities and other public agencies that may be concerned with the acquisition of large tracts of land in and near the city. This is desirable in order that the routes may be properly located for adequate service of the developments planned and that the lands needed for the highways and the new facilities and developments they are designed to serve may be mutually agreed upon and simultaneously and cooperatively acquired.

To illustrate many of the principles of route selection in cities, as well as the range of conditions that may be encountered at cities of various sizes, figure 31 gives schematic lay-outs of several possible conditions of main penetrating and circumferential or distributor routes.

At the small city.—The simplest case is that of the small city, illustrated by diagram A. In this case the interregional highway passes on a direct course wholly without the city. The former main highway which now serves as a city service road, diverges from the interregional route at some distance on opposite sides of the city. Thus it provides a connection between the interregional and the other main highway that passes through the small city. The service road may or may not be considered as part of the interregional system, depending upon the size of the city, its distance from the interregional route, and the relative volume of the traffic the service road and the other main highway contribute to the interregional system. In this case, however, no circumferential or distributing routes are needed.