Page:Interregional Highways.pdf/59

Rh points immediately adjacent to the cities without causing such overlapping of the bands for several highways as to create an undesirable graphical confusion, and in such cases the near-city volumes are not represented at all in figure 20.

Traffic peaks on transcity connections.—To indicate the further increase in traffic volume that occurs when the highways pass into and through cities between the nearest points of recorded observation represented on the two-dimensional traffic map, figure 22 is included. By means of a vertical projection of the traffic bands, figure 22 shows for the recommended interregional system only, what is believed to be a reasonable estimate of the relative magnitudes of traffic volume on all rural sections of the system and on intracity sections at a number of the larger cities.

As suggested by this very approximate picturization, traffic on sections of the routes traversing the cities mounts rapidly to volumes that far surpass the general levels of volume on the rural portions of the system. Moreover, it will be seen from both figure 20 and figure 22 that these rapid increases begin at points comparatively close to the cities.

The peaks represented on the three-dimensional traffic map, figure 22, are in many cases little more than informed guesses; and their sharpness is exaggerated by the unavoidable compression of horizontal scale. That they do not, in fact, exaggerate the relative traffic volumes of the routes within and without the cities, is shown by the comparisons based upon available data for several cities of different sizes, shown in figure 23.

Urban zone of traffic influence.—A study has been made of the available data on traffic flow in the vicinity of all cities of 10,000 or more population directly connected by the recommended system, with the object of determining the approximate distances from each city at which the more rapid increase of traffic volume begins. These distances have been. measured as radial distances from centers located at the heart of the central business areas of the respective cities. They define, for each city, a circular area which may be described as the city’s zone of local traffic influence.

It is found that the radii of these zones tend to increase with the population of the cities. By averaging the radii for all cities of each of several population ranges, the following determination was made of what may be called approximate normal radii of the zones of local traffic influence for cities of different sizes:

Within these zones of local traffic influence around the 587 cities of 10,000 or more population, are 8,141 miles of the recommended interregional system, or 24 percent of the entire system. Of the total mileage within these zones, transcity streets in the cities of 10,000 or