HIGHWAY CONSTRUCTION 125 



Serviceability. The serviceability of a pavement is its quality of 

 fitness for use. This quality is measured by the expense caused to the 

 traffic using it namely, the wear and tear of horses and vehicles, loss 

 of time, etc. No statistics are available from which to deduce the 

 actual cost of wear and tear. 



The serviceability of any pavement depends in great measure 

 upon the amount of foothold afforded by it to the horses provided, 

 however, that its surface be not so rough as to absorb too large a per- 

 centage of the tractive energy required to move a given load over it. 

 Cobblestones afford excellent foothold, and for this reason are largely 

 employed by horse-car companies for paving between the rails; but 

 the resistance of their surface to motion requires the expenditure ot 

 about 40 pounds of tractive energy to move a load of 1 ton. Asphalt 

 affords the least foothold; but the tractive force required to overcome 

 the resistance it offers to motion is only about 30 pounds per ton. 



Comparative Safety. The comparison of pavements in respect of 

 safety, is the average distance traveled before a horse falls. The 

 materials affording the best foothold for horses are as follows, stated 

 in the order of their merit : 



(1) Earth, dry and compact. 



(2) -Gravel. 



(3) Broken stone (macadam). 



(4) Wood. 



(5) Sandstone and brick. 



(6) Asphalt. 



(7) Granite blocks. 



Durability. The durability of pavement is that quality which 

 determines the length of time during which it is serviceable, and does 

 not relate to the length of time it has been down. The only measure 

 of durability of a pavement if the amount of traffic tonnage it will bear 

 before it becomes so worn that the cost of replacing it is less than the 

 expense incurred by its use. 



As a pavement is a construction, it necessarily follows that there 

 is a vast difference between the durability of the pavement and the 

 durability of the materials of which it is made. Iron is eminently 

 durable; but, as a paving material, it is a failure. 



Durability and Dirt. The durability of a paving material will 

 vary considerably with the condition of cleanliness observed. One 



