The last column of the above exhibit shows the relative weights assigned to the quality of cheapness. Since gravel is the lowest in first cost, it possesses the quality of cheapness in the highest degree; and consequently it is given a weight of 15—the value assigned to the ideal pavement in Table 61. The weights assigned to this quality decrease from gravel, the cheapest, to granite block, the most expensive. The several weights assigned above to low first cost are entered opposite this quality in Table 61.
The first cost of a pavement not infrequently has undue weight in comparing the relative merits of different kinds of pavements. In this connection the fact should not be over looked that all the other expenses connected with a pavement— cost of maintaining and cleaning it, of conducting transporta tion over it, of wear and tear on vehicles and horses—is a con tinuing expense, while the cost of construction is incurred once for all; and therefore in comparing the economic value of pavements, it is only the annual interest on the cost of construc tion that should be considered in connection with the other items of annual expense. The pavement which costs the most to construct is not always the most expensive, nor is the one lowest in the first cost always the cheapest in the end. The pavement which is truly the cheapest is the one which gives the most profitable returns in proportion to the amount which is expended upon it, as will be shown under Cost of Mainte nance in § 902.
A pavement is sometimes selected because of its low first cost, for other than economic reasons. Often the cost of construction is charged against the abutting property, while maintenance is paid for by the whole city; and the result is that many property owners prefer a cheap pavement because they must pay for it, notwithstanding the fact that the cheaper pavement may cost more for maintenance and be dearer in the long run. Again, the property holders are sometimes really unable to pay for the most economical pavement, and hence a pavement low in first cost is selected as a temporary expedient.
present state of our knowledge, all that can be done is to make an estimate of the life of the pavement under the particular traffic, and then deduce the annual cost, which includes the interest upon the first cost and the expenditures for repairs including periodical renewals. See § 885.
Table 62 shows the estimate of the annual cost of several kinds of pavements for Minneapolis, Minn.* The computations are made for a term of 20 years for a street having an estimated daily traffic of 150 tons per foot of width. The original table did not contain the last column, which is here added by obvious com putations.
Table 63 gives similar results for pavements in Chicago, by a paving expert.* Interest in this example is computed at 6 per cent.
Table 64, page 587, gives the estimated average cost of several kinds of pavements, computed upon a little different basis than the two preceding tables, by a man having wide experience in paving matters.t "The data assumed in computing this table may be regarded as fair averages for pavements located in cities in the Mississippi Valley, on streets having a traffic of about 33 tons per day per foot of width. The tonnage assumed corresponds to that on a rather heavily traveled residence street, or a business street of medium travel, in cities of 100,000 to 200,000 population, and the result might be entirely different for a street hav ing a larger or a smaller volume of travel. The interest in the table includes that on the cost of construction and on the annual expense for repairs, the latter being computed on the assumption that this expense is uniformly distributed over the last two thirds of the life of the pavement." The original table gives only the total average annual cost of the several pavements during their estimated life, i. e., the original table ends with the eighth line of Table 64; and the ninth line was computed as follows: The difference between the cost of construction and the value of the old pavements was divided by the number of years representing the estimated life of the pavement, and the quotient was subtracted from the total cost per year.