368 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 193 4 



the weight of traffic ; that while it is preserved in a dry state, it will 

 carry any weight without sinking, and that it does in fact carry the 

 road and the carriages also ; that this native soil must previously be 

 made quite dry, and a covering impenetrable to rain must then be 

 placed over it to preserve it in that dry state; that the thickness of 

 a road should only be regulated by the quantity of material necessary 

 to form such impervious covering, and never by any reference to its 

 own power of carrying weight " {8) . It was, therefore. Macadam who 

 first designed a road surface to act as a roof and wearing surface to 

 protect the load-supporting subgrade. This is the fundamental 

 principle upon which modern road design is founded. As a conse- 

 quence the extreme thickness of 3I/2 to 5 feet, used on the great 

 Roman highways, was considerably modified and reduced by road 

 builders in the period following the Renaissance, until in the cross- 

 section of John Loudon Macadam the thickness was reduced to 10 

 inches. A still further reduction in thickness has been made pos- 

 sible by modern methods of road design and construction. In figure 

 6 is shown a high-type rigid pavement suitable for use on a 2-lane 

 superhighway in the United States. This surface is capable of with- 

 standing, for an indefinite period, concentrated wheel loads of 9,000 

 pounds, under motor trucks traveling at the high speed of 50 miles 

 an hour. 



Times have changed. The horse-drawn vehicle, the accepted mode 

 of travel since the dawn of historic time, has been supplanted by 

 mechanical transport. As late as the eighteenth century, research 

 in the field of transportation was directed toward the rediscovery of 

 the lost arts of the ancients. Few new ideas were added to the exist- 

 ing store of knowledge. In the nineteenth century, however, engi- 

 neers on the Continent and in England departed from the beaten 

 track of experience in search of more serviceable types of surface. 

 Thus there were developed the designs of Tresaguet in France and 

 Telford and Macadam in England. These surfaces were adequate as 

 long as the wagon prevailed. With the twentieth century, however, 

 there was introduced a vehicle totally different from any hitherto 

 known. The horseless wagon revolutionized methods of road con- 

 struction and design. Lacking precedents to guide them, engineers 

 were hard pressed to design highways suitable to withstand the 

 destructive effects of the new traffic. 



Nevertheless, out of the maze of observation and research there 

 is being formulated, bit by bit, the new science of highway engi- 

 neering. How far we have advanced may be measured best by con- 

 trasting present conditions with those existing 2,000 years ago. For 

 example, the modern road surface averages roughly one-sixth of the 

 thickness and cost of the ancient Roman road, whereas the present- 



