HIGHWAY COST KEEPING. 11 



UNITS OF MEASUREMENT. 



Care should be taken in selecting the units on which to collect cost 

 data. Too many and varied units will make the system cumbersome 

 and expensive, while too few may impair its value seriously. Fur- 

 thermore, the units of measurement adopted for any cost-keeping 

 system or project must be definite, expressive, readily obtainable, 

 and familiar. Thus, for example, the ton and the cubic yard as 

 applied to broken stone are definite units and afford a ready and 

 accurate comparison, but the square yard when applied to a finished 

 macadam road is indefinite until additional information as to the 

 depth of the material is available. Similarly, many units, such as 

 wheelbarrow, wagon, truck, or carload, while often convenient units 

 of count in the field, are indefinite and always should be reduced to 

 definite comparable units, such as cubic yard or ton. 



The units selected must, so far as possible, be expressive of definite 

 operations. Thus, while in engineering construction the cubic yard 

 is a very common unit upon which contract prices are based, it fre- 

 quently is a very uncertain unit of performance, as it is a composite 

 of other units. For example, in rock excavation there are involved 

 the following operations: (1) Drilling, (2) blasting, (3) breaking large 

 chunks, (4) loading into carts, wagons, cars, or the like, (5) trans- 

 porting, (6) dumping. 



The important item of drilling depends largely on the necessary 

 spacing of the drill holes, which varies in the different kinds of rock 

 and in different kinds of excavation. Clearly, then, the linear foot 

 of drill holes is the unit for measuring the output of the drillers, and 

 not the cubic yard. Transporting the rock is largely a function of 

 distance; hence the unit of transportation cost should be the ton or 

 yard carried 100 feet or 1 mile, and not the cubic yard without the 

 factor of distance. 



The units must be obtainable readily or the cost of collecting the 

 necessary data will be too high. Thus, for example, to obtain the 

 exact cubic yardage and the distance it was moved in preparing the 

 subgrade for a macadam road with a road machine would be not only 

 difficult, but expensive. Hence for this class of work the readily 

 obtainable, though less definite, unit of the square yard usually is 

 adopted. 



That the full value of the cost-keeping system may be realized, 

 the units in which the data are expressed must be familiar to those 

 charged with their collection as well as to those who are to profit 

 from their use. Thus, the cubic meter is as definite a unit for meas- 

 uring earthwork and generally as readily obtainable as the cubic 

 yard, but to the average roadman it has little or no meaning until 

 translated into the terms in which he is accustomed to think. If 



