In the city of medium size.—Diagram B illustrates the case of a city of medium size. In this case a single route of the interregional system approaches the city from the north and south and necessarily passes through the city closely adjacent to the business section to pick up and deliver the substantial volume of traffic there originated or destined.
For the accommodation of the considerable volume of through traffic on the interregional route, a circumferential route, considered as part of the interregional system, diverges to the right at a convenient point south of the city and passes along the eastern boundary to rejoin the main route at a point north of the city. The distance around the city by this route is little if any longer than the distance through the city by the main route. The circumferential route serves also to pick up and deliver traffic at several accesses provided in the city’s eastern quarters.
Another main highway, not included in the interregional system, intersects the interregional route at the center of the city. For transfer of through traffic between this route and the interregional route, a circumferential route is provided around the west side of the city, but because of its relative unimportance in the service of interregional traffic, this route is not considered as part of the interregional system.
In the large city.—Diagram C illustrates the complex pattern of main and circumferential interregional routes and other local belt lines that may be required for the adequate service of both interregional and local traffic at a large city. In this case, three interregional routes intersect at the city and all must pass within convenient reach of the large central business section.
One follows along the bank of the river as it approaches the city and continues in this location through the city.
Another approaches from the northeast and enters the city through a wedge of undeveloped land, then passes on a north–south course along the border of a new housing development, skirts the eastern fringe of the business section, crosses the river, and finally resumes its southwesterly course as it emerges from the city.
The third crosses the city from east to west, skirting the northern edge of the business section.
In addition, several other principal highways center in the city.
In this case, the three interregional routes combine to perform the function of traffic distribution around the business section.
At convenient points to the north, east, south, and west of the city, interregional circumferential routes intersect the main penetrating routes and serve to transfer through traffic from one to another, an to distribute the interregional traffic to the several quarters of the city. The locations of these routes are such that in no case is the distance around the city materially different from the through distance.
To the north of the city there is considerable scattered suburban development, and the northern leg of the interregional circumferential route crosses east and west above all this development.
An additional east–west distributor closer to the city is located as an inner circumferential route approximately along the northern city limits. It connects with the eastern interregional circumferential and with the riverside interregional route. Since it performs mainly a local distributing service, it is not considered as part of the interregional system.
