the type known as the planer will
prove to be the most effective. Figure 16 shows a type of drag that is
very satisfactory for use on gravel roads.

[Illustration: Fig. 16.--Road Planer]

CHAPTER VII

BROKEN STONE ROAD SURFACES

The broken stone road surface, or macadam road as it is usually
termed, consists of a layer of broken stone, bonded or cemented
together by means of stone dust and water. The surface may or may not
be coated with some bituminous material.

=Design.=--It has been an accepted assumption that the macadam road
surface is somewhat more stable than the gravel road surface of equal
thickness, and since this is probably the consensus of opinion of
engineers familiar with both types, it may be accepted until
experimental data are available on the subject.

The thickness of the layer of macadam required for a road will depend
upon the same factors that were considered in connection with the
thickness of the gravel surface, i.e., kind of stone used, character
of earth foundation and nature of the traffic.

The standard macadam surface where good earth foundation is to be had
and where the loads do not exceed about four tons has for years been
eight inches thick. For heavier loads or inferior foundation, a
somewhat greater thickness would be employed, but the best practice
would probably provide a foundation course of the Telford type for
doubtful foundation conditions, especially for the extremely uncertain
cases. For soils of very good supporting strength such as very sandy
loam or deep sand or for arid regions where stable foundation is
always assured the thickness of the macadam might be reduced to six
inches. It should be borne in mind that the broken stone road is not
adapted to the traffic carried by trunk line highways in populous
districts, but is rather a type permissible on secondary roads and
usually adequate for local roads. It should never be employed for
roads carrying any considerable volume of passenger automobile traffic
or motor truck traffic. If surfaced with a bituminous material it will
carry up to 1200 passenger automobiles per day, but not to exceed
fifty trucks.

=Properties of the Stone.=--The stone employed for the broken stone
road should possess the qualities of hardness and toughness and should
be capable of resisting abrasion sufficiently well to have reasonable
life under the traffic to which it is subjected. Since the traffic may
vary from very light on s