operties in a like
degree. The climatic and physical conditions to which roads are
subjected are equally varied. The excellence of a road material may,
therefore, be said to depend entirely on the conditions which it is
intended to meet.

It may be well to mention a few other properties of rock that bear on
road-building, though they will not be discussed here. There are some
rocks, such as limestones, that are hygroscopic, or possess the power of
absorbing moisture from the air, and in dry climates such rocks are
distinctly valuable, as the cementation of rock dust is in a large
measure dependent for its full development on the presence of water. The
degree to which a rock absorbs water may also be important, for in cold
climates this to some extent determines the liability of a rock to
fracture by freezing. It is not so important, however, as the
absorptive power of the road itself, for if a road holds much water the
destruction wrought by frost is very great. This trouble is generally
due to faulty construction rather than to the material. The density or
weight of a rock is also considered of importance, as the heavier the
rock the better it stays in place and the better it resists the action
of wind and rain.

Only a few of the properties of rock important to road builders have
been considered, but if these are borne in mind when a material is to be
selected better results are sure to be obtained. In selecting a road
material the conditions to which it is to be subjected should first be
considered. These are principally the annual rainfall, the average
winter temperature, the character of prevailing winds, the grades, and
the volume and character of the traffic that is to pass over the road.
The climatic conditions are readily obtained from the Weather Bureau,
and a satisfactory record of the volume and character of the traffic can
be made by any competent person living in view of the road.

In France the measuring of traffic has received a great deal of
attention, and a census is kept for all the national highways. The
traffic there is rated and reduced to units in the following manner: A
horse hauling a public vehicle or cart loaded with produce or
merchandise is considered as the unit of traffic. Each horse hauling an
empty cart or private carriage counts as one-half unit; each horse, cow,
or ox, unharnessed, and each saddle horse, one-fifth unit; each small
animal (sheep, goat, or hog), one-thirtieth unit.