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102 can therefore be obtained at approximately minimum cost, that the time is now ripe for the undertaking of such improvements.

The belt line, shown as encircling the city, would be a limited-access facility, with all intersecting highway grades separated and access provided only from the more important roads. It is probable that the greatest single contributions to this belt line would be those of US 1 at the south of the city and U S 40 at the east. Between these two highways the belt line would serve as a bypass route for that part of the Atlantic coastal movement that is desirous of avoiding the city entirely. The same section, including, as it does, a bridge across the city’s outer harbor, would serve to connect two rapidly growing industrial sections on opposite sides of the river.

Because of the probably heavy volume of turning movement at the intersections of the belt line with U.S. 1 and 40, a special Y connection at these points might be desirable. Such a connection with U.S. 1 at the south of the city is illustrated in plate 55. In this sketch the belt line is shown in the middle distance coming from the west and continuing eastward toward the southern riverside industrial suburbs. In the center of the picture is shown the Y connection with U.S. 1, and the route of the belt line crossing the distant river toward the northeast.

DIRECT INTERREGIONAL ROUTES AND MODERNIZED RURAL HIGHWAYS

Beyond the vicinity of cities the existing main rural highways of the United States lack a sufficient capacity to discharge the flow of present traffic moving over them with reasonable convenience only at relatively few points.

In the discussion of routes to be operated as toll facilities (see p. 40) a traffic of 1,500 vehicles was adopted as the criterion for determining whether a two-lane or a wider pavement was required. Average daily traffic in volumes less than that amount it was assumed would require no more than two lanes for its free movement; traffic exceeding that average was assumed at some time during the year to produce a maximum hourly flow that would be slightly inconvenienced unless more than two pavement lanes were provided.

It was pointed out that a toll road, for use of which a special fee is asked, would have to provide a capacity that at any time would not be likely to limit the freedom of motion of its users. So high a standard of service is not to be expected in a system of roads built with general taxes. On such roads a slight restriction of absolute freedom of movement is to be expected during the few short periods of maximum hourly traffic volume that occur in the course of a year. So long as such occasional restrictions do not result in the creation of a dangerous condition or in substantial congestion or retardation of the traffic they may be accepted as reasonable. On this basis, an average daily volume of 2,000 vehicles may be considered as within the reasonably convenient discharge capacity of a two-lane highway.

Accepting this figure as a criterion, the highway planning survey data for 12 representative States, presented in table 20, show that in these States there are only some 4,651 miles, or 0.6 percent of their total of 766,314 miles of rural highways on which the traffic is of such density as possibly to tax the capacity of a two-lane pavement to provide reasonably convenient service. On January 1, 1938, there were