"The size to which stone should be broken depends upon the quality of the stone, the amount of traffic to which the road will be subjected, and to some extent upon the manner in which the stone is put in place. If a hard, tough stone is employed it may be broken into rough cubes or pieces of about one and a half inches in largest face dimensions, and when broken to such a size the product of the crusher may generally be used to good advantage without the trouble of screening, since dust 'tailings' and fine stuff do not accumulate in large quantities in the breaking of the tougher stone.
"If only moderate traffic is to be provided for, the harder limestones may be broken so the pieces will pass through a two-inch ring, though sizes running from two and a quarter to two and a half inches will insure a more durable roadway, and if a steam roller is used in compacting the metal it will be brought to a smooth surface without much trouble. As a rule, it may be said that to adhere closely to a size running from two and a quarter to two and a half inches in largest face dimensions, and to use care in excluding too large a proportion of small stuff as well as all pieces of excessive size, will insure a satisfactory and durable macadam road."
Macadam insisted that no large stone should ever be employed in road-making, and, indeed, most modern road builders practice his principle that "small angular fragments are the cardinal requirements." As a general rule it has been stated that no stone larger than a walnut should be used for the surfacing of roads.
Stone roads are built in most cases according to the principles laid down by John L. Macadam, while some are built by the methods advocated by Telford. The most important difference between these two principles of construction relates to the propriety or necessity of a paved foundation beneath the crust of broken stone. Telford advocated this principle, while Macadam strongly denied its advantages.
In building roads very few iron-clad rules can be laid down for universal application; skill and judgment must be exercised in designing and building each road so that it will best meet the requirements of the place it is to occupy. The relative value of the telford and macadam systems can most always be determined by the local circumstances, conditions, and necessities under which the road is to be built. The former system seems to have the advantage in swampy, wet places, or where the soil is in strata varying in hardness, or where the foundation is liable to get soft in spots. Under most other circumstances experienced road builders prefer the macadam construction, not only because it is considered best, but also because it is much cheaper.