MODERN ROAD PRACTICE Types of Road.—Prior to 1904, the major types of surfacing were gravel and macadam, which gave entire satisfaction under the normal traffic of relatively light horse-drawn, steel-tired ve hicles, with a bicycle traffic near the cities. The outpouring of motor vehicles from the cities which began about 1904 caused the macadam roads to "ravel," and maintenance under such traffic was impossible. Tars and asphalts were substituted for the weaker binders; first, as dust layers, as protective surface coatings, and then as binders introduced into roads of the macadam type by penetration, and finally as hot admixtures according to the bitu minous concrete principle. These types were entirely satisfactory for automobile traffic.
From 1904 to 1914 is the period of bituminous construction. In 1904, there were in the entire country only 18 miles of bitumi nous rural roads, all in the two States of Massachusetts and Ohio. By 1914, there were io,5oo miles, a mileage which was nearly three-quarters of the aggregate length of all roads of higher type than macadam. The decline in the surface-treated and penetration types of macadam began when motor trucks in considerable num bers began to appear on the rural highways. They brought a demand for rigid pavements of concrete and brick and bituminous concrete on a concrete base.
Although the first concrete road had been built in 1893, in Bellefontaine, 0., there were no more than five miles of that type on rural highways in the entire country in i9o9. The first big increase occurred in when more than 25o miles of rural high ways were paved, to be followed in 1913 with soo miles, and in 1914 with more than 1,500 miles. At the close of 1914, there were in the entire country 2,348 miles of concrete roads. Ten years later the mileage had increased to 31,186 and construction was proceed ing at the rate of more than 6,000 miles a year, a rate approached by no other type better than gravel.
The increase of motor trucks on the highways also caused a more extensive use of brick, and the bituminous pavements of the mixed type on concrete bases. In 1914 there were approxi
mately i,600 miles of brick pavement ; in 1924 there were 4,319 miles. In 1914, the bituminous concrete or sheet asphalt mileage on rural highways was negligible ; in 1924 there were more than 9,70o miles of these types.
Current Policies and Practices in Road Construction.— The main highways are being improved with high-type surfaces such as brick, concrete, and bituminous mixtures. For moderate and light traffic waterbound macadam, gravel, sand-clay, selected soil mixtures, and low-cost bituminous mixtures are used. Practi cally all surfaces that tend to become dusty in dry weather are made dustless by applying a bituminous treatment during con struction or soon thereafter. Great progress has been made in learning how to combine granular materials such as sand and gravel with clay and bituminous binders to form durable surfaces that are moderate in cost.
The kind of soil on which a road surface is placed has a great deal to do with how the surface lasts. Engineers are now able to test soil samples taken from the location where a highway is to be built and determine where the soil will give good support, where the surface must be thickened for additional strength, and where poor soil must be replaced by better material.
Until about 193o road building efforts were directed largely toward the extension of surfaced mileage. Such a policy was nec essary so long as sections of unsurfaced roads remained on princi pal routes of travel. Most of the gaps are now closed. The ob jective fixed many years ago has been reached, but the amount of needed highway improvements is greater than ever before. The great increase in volume and speed of traffic has made obsolete the highways built years ago when conditions were different. Ex isting highways are being modernized by placing better and wider surfaces, eliminating sharp curves and steep grades, eliminating railroad grade crossings, and placing warning and danger signs and marks. Safety of travel is an important consideration in this work.