Page:America's Highways 1776–1976.djvu/420

 Aware of the need for uniformity of State motor vehicle weight regulations, the Governors Conference, in 1949, requested a study of the matter. This study resulted in a rigid pavement test road being established in Maryland, with participation by 11 State highway departments, the District of Columbia, the Bureau of Public Roads, auto manufacturers and the petroleum industry. The purpose of the testing program was to determine the relative effects of various axle loads and configurations on distress of rigid pavement. The findings supplied extensive data on the factors to be used in the design of rigid pavements. Especially significant were those factors regarding support material characteristics.

The Western Association of State Highway Officials constructed a test road in Idaho in 1951 to aid in establishing load limits and to develop rational design methods for flexible pavements. A number of specially designed and constructed bituminous pavements were carefully observed under the repeated application of a number of selected heavy axle loads. The findings provided significant information on the materials and soil parameters for designing flexible pavements, especially in the western States where soil conditions are similar. Also the surfaced shoulder was found to contribute to the pavement structural stability.

In 1955, the American Association of State Highway Officials undertook the AASHO Road Test at a selected site near Ottawa, Illinois, with the Highway Research Board accepting the responsibility of administering the project. Various heavy truck loads were operated on specially constructed pavement sections of both rigid and flexible types until they reached a failure stage. A vast amount of data was collected and analyzed, providing engineering facts for highway design and construction nationwide. In addition, the test findings were aimed at determining maximum desirable weights of vehicles to be operated on Federal-aid highways, including the Interstate System, and determining an equitable distribution of the tax burden among various classes of persons using Federal-aid highways.

The findings of the AASHO Road Test were summarized and prepared in the form of design equations and graphs and made available to the States in 1962, but it was not until 1973 that the data was updated and published by AASHO as Interim Guide for the Design of Rigid Pavement Structures. The equations and graphs incorporated the research data for design factors such as traffic, soil support and material strengths. The guide today represents the major current data available for broad application in designing pavements and is used by the Federal Highway Administration to measure the adequacy of the States’ proposed pavement designs for use on Federal-aid highways.

Present day pavements consist of layers of bituminous materials or portland cement concrete, plain or reinforced, with thicknesses ranging up to 10 inches; in some instances, unusually high truck loadings call for even greater thicknesses. Most pavements today include a subbase which is a stratum of material 4 to 20 inches thick between the pavement and natural subgrade. This subbase is a granular material sometimes treated for stabilization with either cement or asphalt, Design criteria, which account for soil support values, material strength characteristics and traffic, are used to determine the thickness of the pavement subbase.

The records of the development of roads and streets in the United States show a progressive increase in the total mileage and a continued conversion from the lower to higher surfacing and pavement types. The actual rate of development and change reflects not only the increase in funding for public highways to serve the rising traffic needs, but also the findings from research studies on surface materials, aggregates, reinforcements, mixes, construction and maintenance.

In 1904 there were about 2.35 million miles of public streets and highways. From 1910 to 1920 the total increased rapidly to about 3.2 million miles in all stages of improvement, ranging from primitive trails to highly improved urban thoroughfares. In 1920, 425,000 miles had some form of surfacing. Since 1950 the total mileage has increased consistently to the 1973 total of 3.8 million miles. The urban mileage has been expanding somewhat more rapidly than the rural mileage, particularly since 1950.

About 90 percent of the total road mileage was not surfaced in 1904. During the total mileage expansion in the teens, the nonsurfaced mileage actually increased, but the construction programs resulted in a gradual proportional decrease to about 80 percent in 1925. Since then the nonsurfaced mileage has decreased rapidly to about 20 percent in 1973.

Since about 1920 the total surfaced mileage has been increasing steadily to the 1973 total of 3.17 million miles. The gravel surfacing type has been dominant. Since about 1935 the rate of surfacing urban highways has been somewhat more rapid than that for the rural mileage.

The mileage of rural gravel-type surfaces (soil, slag, gravel and stone) increased regularly to about 1960. Since 1962 this type has been decreasing. It should be noted that the increase in gravel mileage up to 1960 was over and above the mileage that was upgraded to higher surface types. From 1935 to 1955 the rural gravel improved mileage was from 3 to 4 times that of the low bituminous mileage. The record on urban gravel mileage shows little change from 1941 to 1973, with a continuing total of 70,000 to 80,000 miles.

The low bituminous rural mileage jumped substantially during the 1930's and increased steadily until the late 1960’s, when it leveled off. In the urban areas, the low bituminous mileage continually increased.

The high bituminous rural mileage followed the upward trend of low bituminous type, with totals only about half to two-thirds that of the lower type. The decided increase about 1950 corresponds with the decrease in the low bituminous type. This type continued the upward trend the last few years, as distinct from the low bituminous mileage,

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