Notice that some of the above results include only vehicular traffic and others include both vehicles and horses. No systematic observations have ever been made to determine the relative destruc tiveness of horses and vehicles; but apparently, compared ton for ton, horses are more destructive than vehicles. The sharp blows of horses' shoes, particularly if they have heel and toe calks, are very destructive to stone-block and brick pavements, as they spall off the edges of the blocks.
Modifying Elements. Although the effect of traffic is dependent chiefly upon the number of tons per foot of width, its influence is modified somewhat by (1) the character of the pave ment, (2) the state of repairs, (3) the degree of cleanliness, (4) the presence or absence of car tracks, (5) the width of pavement, (6) the character of the traffic, and (7) the climate.
1. The durability of a particular kind of pavement will vary with the details of the method of construction. The foundation may be more or less rigid, the materials may differ greatly in durability, with any form of block pavement the joints may be more or less open, and the surface may also vary more or less in roughness.
2. The durability will depend upon the care employed n re pairing the pavement. If holes, depressions, or ruts are allowed to remain for any length of time, whatever the material the pave ment will wear abnormally fast.
3. The degree of cleanness will materially modify the durability of a pavement. An imperishable material is benefited by a cov ering of detritus, since it serves as a carpet to protect the pave ment; and if the covering is heavy enough the pavement virtually becomes a foundation and is entirely protected from wear. On the other hand, the decay of a perishable material, as wood and asphalt, is hastened by a covering of street dirt which collects moisture and hastens the decay and disintegration of the pavement.
4. The presence of a street-car track on a street concentrates traffic at the two sides, thus virtually narrowing the street, and also causes the traffic to go substantially in one track, a result which is particularly destructive of gravel and macadam roads.
5. The wider a pavement the more evenly will it wear, and consequently the longer it will last. If several irregular lines of travel can be maintained, the wear will be much more even and the durability greater than if the vehicles are restricted to prac tically a single line.
6. The durability of the pavement will vary with the weight per unit width of tire, the method of shoeing the horses, and the rapidity of the traffic. In Europe, the weight per unit of width of tire is generally regulated by law, and calks on the horses' shoes are prohibited; but in America there are no such restrictions. Rapid traffic is more destructive to a block pavement than slow traffic.
7. The climate affects the durability of several kinds of pave ment. The durability of a wood pavement is affected by the heat
and the moisture conditions, that of macadam and gravel by moisture and winds, and that of asphalt by moisture, particularly by street sprinkling. Sprinkling materially affects the durability of wood pavement, as is shown where a strip is ordinarily left un sprinkled for a foot-way.
It is generally conceded that granite block is the most dur able paving material, especially under very heavy loads.
Asphalt and brick rank next to granite, and when well con structed give excellent service except perhaps under the very heaviest traffic; although it should be noted that as the method of preparing and laying these materials becomes better understood, they are being laid under heavier and heavier traffic. For example, within the last year or two artificial sheet asphalt has replaced granite block on Broadway in the most congested district of New York city—a street probably having the most traffic of any in this country (see Table 57, page 568). Again, only a few years ago it was considered that bricks were suitable only for small cities; but now Philadelphia, the third largest city on this continent, has 128 miles of brick pavements. The production of granite paving blocks has decreased one half in the past ten years, apparently because of the introduction of asphalt and brick pavements. The relative durability of brick and asphalt is a matter of doubt, both materials showing varying results due to differences in the quality of the material and to the method of construction. The following examples show the possible durability of these two materials. A brick pavement on concrete with cement-filled joints laid on one of the principal business streets in Terre Haute, Ind., after eleven years' service without repairs, showed a maximum general wear of only to 1/32of an inch with a few holes showing a wear of of an inch, the population of the city increasing in the meantime from 30,000 to 36,600 and there being a double track car-line on the street. An asphaltic limestone pavement on Cheapside, London, which has a daily traffic of 290 tons per foot of width (see Table 58, page 571), wore down about 1 inch in seventeen years. It is said that the average life of asphaltic limestone pavements in London and Paris is about seventeen years. An artificial sheet asphalt pavement on Pennsylvania Avenue, Washington, D. C.. was re-laid after thirteen years' use. The method of repairing asphalt pavements, both artificial and natural rock, may, however, make such examples as the last two misleading. For data on cost of maintenance of asphalt pavements, see § 670-74.