A record is made of the traffic every thirteenth day throughout the year, and an average taken to determine its mean amount. Some such general method of classifying traffic in units is desirable, as it permits the traffic of a road to be expressed in one number.
Before this French method can be applied to the traffic of our country it will be necessary to modify considerably the mode of rating. This, however, is a matter which can be studied and properly adjusted by the Office of Public Road Inquiries. It is most important to obtain a record of the average number of horses and vehicles and kind of vehicles that pass over an earth road in a day before the macadam road is built. The small cost of such a record is trifling when compared with the cost of a macadam road (from $4,000 to $10,000 per mile for a fifteen-foot road), in view of the fact that an error in the selection of material may cost a much larger sum of money. After a record of the traffic is obtained, if the road is to be built of crushed rock for the first time, an allowance for an immediate increase in traffic amounting at least to ten or fifteen per cent had best be made, for the improved road generally brings traffic from adjoining roads.
To simplify the matter somewhat, the different classes of traffic to which roads are subjected may be divided into five groups, which may be called city, urban, suburban, highway, and country road traffic, respectively. City traffic is a traffic so great that no macadam road can withstand it, and is such as exists on the business streets of large cities. For such a traffic stone and wood blocks, asphalt, brick, or some such materials are necessary. Urban traffic is such as exists on city streets which are not subjected to continuous heavy teaming, but which have to withstand very heavy wear, and need the hardest and toughest macadam rock. Suburban traffic is such as is common in the suburbs of a city and the main streets of country towns. Highway traffic is a traffic equal to that of the main country roads. Country road traffic is a traffic equal to that of the less frequented country roads.
The city traffic will not be considered here. For an urban traffic, the hardest and toughest rock, or in other words, a rock of the highest wearing quality that can be found, is best. For a suburban traffic the best rock would be one of high toughness but of less hardness than one for urban traffic. For highway traffic a rock of medium hardness and toughness is best. For country road traffic it is best to use a comparatively soft rock of medium toughness. In all cases high cementing value should be sought, and especially if the locality is very wet or windy.