For the various reasons mentioned, the traffic congestion that exists on a main rural highway at the approach to a city is usually augmented on the connecting city street toward the center of the city and continues on the same or even another street to the continuation of the rural route at the other side of the city.
The remedy commonly proposed for these conditions is the construction of a bypass highway. It is inaccurately assumed that the congestion results from the joining of the local with the through traffic, and that a substantial relief would be obtained if the through traffic were diverted at a point outside the city beyond the beginning of congestion, and carried on a bypass to a similar point on the rural route at the other side of the city. In rare cases this remedy alone may prove sufficiently effective, but, as hereafter elaborated, bypass routes are of advantage mainly to a relatively small part of the highway traffic normally approaching a city, i. e., to that small part of the traffic that is actually desirous of avoiding the city.
As all traffic maps show, the greater part of the heavy traffic at a city entrance is an in-and-out movement of local generation. That part cannot be drained off by a bypass route. Of the remaining traffic of longer range, there is a further considerable part destined to, or originated in the city, that also would not use a bypass route if one were offered.
An idea of the relation that may exist, on a main road approaching large cities, between the bypassable through traffic and the traffic that is desirous of entering the city and cannot be diverted by a bypass route may be obtained from plate 49. This plate shows a traffic profile of the highway between Washington, D. C. and Baltimore, Md. As shown by the topmost line in this graph, the total traffic on the route rises to a peak at each city line and drops to a trough between the two cities. Of this total traffic, that part above the highest of the horizontal lines represents movements of less length than the distance between the cities. At each city line this part consists of movements into and out of the city all of which are of shorter range than the distance to the neighboring city. The uniform vertical distance between the highest and next lower horizontal line measures the amount of traffic on the road moving between points in each city. The height of the next lower horizontal band represents the traffic moving between Washington and points beyond Baltimore; that of the next, the traffic moving between Baltimore and points beyond Washington; while the height of the lowest horizontal band measures the volume of the traffic moving between points that lie beyond both Baltimore and Washington. Of all the traffic shown as entering the two cities, only this last part plus that represented by one or the other of the next two higher bands can be counted as potentially bypassable around the two cities. At Washington this bypassable maximum is 2,269 of a total of 20,500 entering vehicles; at Baltimore it is 2,670 of a total of 18,900 vehicles. The remainder of the entering traffic in each case will not only continue into, but in large part will penetrate to the very heart of the city, because that is where most of it is destined, and conversely it is at or through the same center that one must look for the source of most of the city-originated emerging traffic.
