An examination of the three preceding tables shows that the life of the pavement, or the cost of perpetual mainte nance, is the most important matter in comparing the relative economy of two or more pavements. The estimated relative degree in which the several pavements in Table 61 possess the desirable quality of low cost of maintenance is shown by the percentages in line 2 of that table.
Under this may be included not only the power required to move loads but also the con sequential damages to vehicles, since they both vary with the roughness of the pavement. From a study of the results in Table 9, page 31, the weights are assigned to this quality for the different kinds of pavements, as shown in Table 61.
From a study of § 887, the relative degree of slipperiness is stated in numbers and entered in Table 61. If the pavement is to be upon a steep grade, this quality may be a controlling factor.
The relative ease with which certain types of pavements may be swept, as determined by the cost of doing the work in New York city, is as follows: asphalt, 100; brick, 100; rectangular hard-wood blocks, 100; granite blocks, 150; Belgian blocks, 160; cobble stones, 400.* • For sanitary reasons, New York city has spent a million dollars a year for the past few years in substituting sheet asphalt pavements for stone block in the congested tenement districts, chiefly on account of the greater ease with which the asphalt is kept clean.
The cost of sweeping ordinary stone block, round wood block, and brick with sand filler usually ranges between 40 and 48 cents per 1,000 square yards for each sweeping, and sheet asphalt from 30 to 38 cents, depending upon the thor oughness of doing the work, the frequency of sweepings, the kind of business in the property adjoining, and the amount of the traffic. The relative weight to be assigned to this item will
vary with the frequency of cleaning.
The estimated weight to be assigned to the several pavements on account of their ease of cleaning is entered in Table 61, page 583.
From a consideration of the discussion in § 892-96, the percentages for the other qualities are inserted in Table 61.
The totals at the foot of Table 61, page 583, represent the summation of the individual decisions on the several qualities, and the larger the total the more desirable the pavement. The particular results in this example may not be applicable to any locality, and each person will have his own opin ion as to the merits and defects of any particular pavement; but the method of analysis is applicable to any particular case, and will enable the engineer intelligently and unerringly to reach the final conclusion to which his opinion in detail leads. The above method has something of the mathematical form, but the fact should not be forgotten that it is based upon judgment and that therefore it can not be expected to give results of a high degree of accuracy.
In practice the application of this method is much less com plicated than appears from the above example, for usually prox imity of some natural pavement materials or freight rates on others, limits the choice to a comparatively few kinds of pavements. Further, the decision as to the kind of pavement to be laid is often influenced by the fancy or ability of those who pay for it. How ever, the engineer should employ a logical process in arriving at his own conclusions, and thus be in a position to give sound advice upon the economic principles involved.