Citation

Gardner EH. Highw. Res. Board bull. 1960; 249: 1-22.

Copyright

(Copyright © 1960, National Research Council (U.S.A.), Highway Research Board)

DOI

unavailable

PMID

unavailable

Abstract

Conventional sufficiency ratings are subjective and arbitrary in the assigning of point values, and fail in the comparison of rural with urban facilities. A proposed priorities rating formula for the commonwealth of pennsylvania, based on rational sufficiencies, resolves most of these difficulties, and is arbitrary only in accepting desirable speeds as 50 mph (legal) in rural and 30 mph in urban areas. The formula does not use safety as an independent factor, believing its containment in the structural and functional elements of rating to be a proper evaluation and not to be duplicated within a rating system. Non-uniformity of accident reporting, and non-separability of driver psychology from road characteristics as accident causation further determined this decision. Structural and functional factors are evaluated in dates of retirement rather than points. Date of structural obsolescence is found from the survivorship curves of the bpr road life study, utilizing the area under the curves between 1959 and expiration to determine the average life of the unretired mileage of the applicable road surface. The date obtained is correctible by field observation of visible abnormal failures. Functional obsolescence is determined by the calculated year in which forecasted demand volumes equal the capacity of the road section. Capacity of rural roads is computed by the method described in public roads, june 1958, using pennsylvania's policy speeds. Comparable urban capacities were not available, necessitating a research study which found average capacities of city streets at a desired speed of 30 mph. Because of deferred construction, a significant portion of mileage is currently operating above capacity. An additional technique to determine priority for these road segments calculates the total vehicle delay, using curves of the aforementioned method and the urban study to find average travel speeds for varying volumes of using traffic. All hourly volumes exceeding the hourly capacity volume are analyzed for vehicle-hour delays and accumulated. Hourly percents of adt's are necessary to the computations, which require the updating of records from pennsylvania's 55 permanent count stations. Charts of these findings are contained in the paper. A modified benefit-cost ratio is obtained by dividing total vehicle-hours delay cost (which represents a relative measure of benefit to be achieved by reconstruction) by the estimated cost of the improvement. This ratio indicates the congestion that can be alleviated per dollar expenditure. Road sections are tabulated by years of obsolescence, and in descending values of this ratio, insuring that appropriations will be expended to alleviate the greatest amount of vehicle delay. The needs study by years is thus unfolded, and, balanced against appropriations, a construction program is established.