AI'I'I.H'ATIoN OK KKSl'I/rs. 43 



results will be resvrved 1'or a special report on the eenient ing \:rluc of 

 road materials, to he published as soon as possihle. 



It has alreaxly been stated that a road materi-al can not lie selected, 

 irrespective of the volume and character of trallic and t he cJimat ic 

 conditions to which it is to be subjected, without the risk of failure. 

 When the very great cost even of rural highways is considered, it is 

 ev'mlent that it is gross negligence not to use every preliminary pre- 

 caution to guard against expensive mistakes. In order to properly 

 designate the conditions best suited for a particular material it soon 

 became evident that traffic must be classified according to its volume 

 and character. The following live divisions of trallic are therefore 

 made: City (perurban), urban, suburban, highway. a.nd country road. 

 City (perurban) traffic is such as exists on the business streets of hunjv 

 cities, and no macadam road can withstand it. For such a trallic stone 

 and wood blocks, asphalt, brick, or some such materials are necessary. 

 Urban trallic is such as exists on city streets which are not subjected 

 to continuous heavy teaming, but which have to withstand very heavy 

 wear, and require the hardest and toughest macadam rock, or other 

 highly resistant material. Suburban traffic is such as is common in 

 the suburbs of a city amd the main streets of country towns. High- 

 way traffic is a traffic equal to that of the main country roads. Country 

 road traffic is that of the less frequented country roads. 



This classification is purely arbitrary, but it serves the purpose for 

 which it was intended, and each group can be approximately identified 

 by a road builder. City (perurban) traffic requires the hardest and 

 toughest materials available, having the highest wearing qualities. 

 For urban traffic, such materials as asphalt, brick, wood block, and 

 bituminous macadam are suitable, and if ordinary macadam is used, 

 a rock of the highest hardness, toughness, and wearing qualitie- i- 

 needed. For a suburban traffic the best rock would be one of high 

 toughness but of less hardness than for urban tra.flic. For highway 

 traffic a rock of medium hardness and toughness is best, awid for 

 country road traffic a comparatively soft rock of medium tbughne . 



To obtain the best results on a macadam road that rock should be 

 selected which would resist the wear of the traffic to which it is sub- 

 jected to such a degree as to supply just the sufficient amount of bind- 

 ing material to cement the road. Too much or too little wear are 

 alike injurious. The higher the cementing value of the rock the 

 smaller the wear necessary. When a road is first constructed a 

 sufficient amount of binding material must be supplied to cause it 

 to "come down" under the roller or traffic. If the subsequent traffic 

 is not sufficient to wear off the requisite amount of binder to replace 

 that carried off by wind and rain the road "ravels." If the traffic 

 wears off an excess of binding material, mud and dust result. In 

 either case the material is not well suited to the conditions. In 



