Round-block soft-wood pavements are lacking in durability, but rectangular blocks of both soft and hard wood have given satisfactory service under heavy traffic in London and Paris. For example, at the end of Westminster Bridge, where the traffic from 6 a. m. to 6 p. m. is 334 tons per foot of width, of which 15 per cent is heavy omnibuses. the mean of six years' wear of jarrah rectangu lar blocks was 0.16 inch per year.* On Euston Road, London, where the traffic not including horses was 529 tons per 24 hours per foot of width, of which 7.7 per cent was heavy omnibuses, the maximum wear of the jarrah and karri rectangular blocks was 0.08 of an inch per annum and yellow deal 0.46 of an inch.t At another place on Euston Road, where the traffic, not including horses, was 381 tons per 24 hours per foot of width, the maximum wear of jarrah blocks was of an inch, the minimum being of an inch, and the average inch./ Broken stone and gravel wear rapidly under moderately heavy traffic, and are suitable only for residence and suburban streets, and for park roads and pleasure drives.
In trying to determine the probable life of a pavement, two facts should not be overlooked, viz., (1) The average wear does not determine the life of a pavement, since even the most care fully constructed pavements wear so unevenly as to require re-lay ing before the wearing coat is entirely worn out. This is true of macadam and sheet asphalt which have a comparatively thin wearing coat, and is particularly true of pavements made of blocks, as wood, brick, and stone, since the edges of the blocks wear off and leave the top face rounded, and when the pavement reaches this stage the wear is much more rapid than previously. (2) In a block pavement the blocks must have a certain depth to enable them to keep their place, and consequently bricks and shallow wood blocks can not be worn more than about half-way through. If the blocks are made deeper, the durability of the pavement is not increased much, if any, since owing to unequal wear the pave ment must be re-laid before any considerable depth is worn off. Asphalt and trap-topped macadam have some decided economic advantages over other forms of pavements, since the wearing sur face consists of a comparatively thin wearing coat that can be re placed when it is worn out or wears rough, without proportionally as much loss as when a block pavement is re-surfaced. A further economic advantage of these pavements is that when holes begin to form a patch can be applied and thus the uniformity of the surface may be preserved and the life of the pavement be extended. Trap-topped macadam has an economic advantage over asphalt, in that when it is re-surfaced the old material is not thrown away but is simply picked loose and mixed with the new stone.
Many attempts have been made to compute the financial advan tage of a decreased tractive resistance, but it is impossible to deter mine its value with any degree of accuracy, although it is certain that the tractive resistance of the pavements of a city are impor tant factors in determining the cost of conducting transporta tion. Ease of traction is, however, not relatively as important for city pavements as for country roads, since in the latter ease of traction is a matter of first importance (see § 4-9), while in the former it is comparatively unimportant (see § 442). On the other hand, the cost of transportation per ton-mile is considerably more in the cities than in the country.
Slipperiness.The method of comparing pavements in this respect is to determine the distance a horse travels on the different pavements before he falls. The most complete observa tions made in the United States to ascertain the prevalence of accidents on the different pavements were made under the direc tion of Capt. F. V. Greene.* The observations were made from 7 a.m. to 7 p.m. on six consecutive days in October and Novem 'y2r, 1885, in ten of the leading American cities on thirty-three streets having the heaviest traffic for each kind of pavement in the particular city. The number of horses observed on asphalt pavements were 360,254, on granite 376,384, and on wood 70,914; and the number of miles traveled by the horses while under observation was 41,427 on the asphalt pavements, 34,723 on the granite, and 4,901 on the wood. A summary of the results is shown in Table 59.
An elaborate series of observations was made in Lon don in 1873 by Col. William Haywood.t The three classes of pavements, asphalt, granite, and wood, were observed as nearly as possible under the same conditions of space, weather, gradi ents, etc., on fifty different days. The results are shown in Table 60.