In the first row of plottings above the pavement and right-of-way bands the height of the narrow black vertical bars, each of a width corresponding to a distance of 1 mile to the horizontal distance scale, represents to the indicated scale the number of curves in each mile of the highway that in 1937 were sharper than certain indicated desirable standards, generally 6° in nonmountainous areas and 14° in mountainous areas.
In the next higher row, the narrow vertical bars represent to the indicated scale, the number of grades, longer than 500 feet, in each mile exceding 5 percent in nonmountainous areas and 8 percent in mountainous areas, considered generally as desirable maximum limits.
Similarly, the height of the vertical bands in the next row represents to scale the number of sight distances in each mile shorter than desirable limits of 1,000 feet and 650 feet in nonmountainous and mountainous areas, respectively; and those in the next row in like manner represent the number of fatal highway accidents occurring on each mile during the year 1937.
At the top of the diagram the average daily volume of traffic on each mile of the highway is shown by three profiles, the highest representing the total vehicular traffic and the two lower ones, respectively: the traffic of motor trucks and busses and the so-called “foreign” traffic which, in each State, consists of all vehicles bearing license tags of other States.
Vertical lines extending through all of the data rows of the diagram, show to the horizontal scale the sequential location of the corporate limits of cities and State borders.
Examination of this highway condition diagram shows instances in which variations in the width or type of pavement or both are inconsistent with corresponding variations in the average daily traffic and a number of sections on which pavements of two-lane width are provided for traffic exceeding an average daily volume of 2,000 vehicles.
The existing rights-of-way are shown to be in the main quite narrow, generally less than 100 feet in width; and there is a marked absence of either uniformity or consistency in the widths provided. Wide pavements are confined within relatively narrow rights-of-way and narrow pavements in other places are laid on relatively wide rights-of-way. In some places the available right-of-way will be observed to constitute a definite impediment to desirable widening of the pavement.
Unsatisfactory conditions in respect to sight distance, grade, and curvature are shown to be of common occurrence; and on some sections the conjunction of such unsatisfactory physical conditions of the highway with a bad accident record suggests the possibility that the highway conditions may be in some measure responsible for the fatal accidents that have occurred. In other instances the record of highway physical conditions, including the width of pavement, appears to clear the highway of responsibility for accidents that have occurred.
The diagram reproduced in plate 56 is typical of others that have been drawn for nearly 27,000 miles of important main highways in the United States; and the conditions shown on this diagram are fairly indicative of those that exist generally on all the main highways of the country. There is no doubt that, as measured by the standards of the diagrammatic record, unsatisfactory conditions with respect to sight distance, curvature, and gradient are common. There
