Requirements of an Ideal Pavement

The average distance traveled by a horse before an accident occurred was as follows: As a result of the above observations, the following conclu sions were drawn. " Slight rain makes asphalt and wood more slippery than they are at other times. On asphalt the slipperi ness begins almost immediately after the rain commences, while wood requires more rain before its worst condition ensues. The slipperiness lasts longer upon wood, on account of its absorb ent nature, than it does upon the asphalt. When dry weather comes after the rain, asphalt is in its most slippery condition and horses fall upon it very suddenly. Wood is frequently in that peculiar condition of surface in which horses slip or slide along without falling. A small quantity of dirt on asphalt makes it very slippery. In damp weather granite blocks be come very greasy and slippery; in dry weather, if they are of a hard variety, the surface polishes and becomes rounded and the only foot hold is by the joints between the blocks." The difference in the results for slipperiness of pave ments in London and in American cities may be due in the case of the wood and the stone pavements to climatic causes. London is more damp and foggy than any one of the American cities in which the traffic was observed, and therefore its pave ments would be more slippery. The difference in the case of asphalt may be accounted for by the difference in the character of the material. The asphalt pavements in London are made from asphaltic limestone, which makes a very smooth, hard surface; while the American pavements are made from natural bitumen mixed with sand, which forms a rough, granular sur face. Further, in London, and generally in Europe, the horses' shoes have no calks, and therefore they will slip more than in America where shoes with calks are the rule.

The slipperiness of a pavement varies greatly with the de gree of its cleanliness. The slipperiness of an asphalt pavement can be decreased by sprinkling coarse sand over the surface, and the slipperiness of wood can be greatly decreased by strew ing small pebbles over it, both of which remedies are frequently used in London and Paris.

No observations similar to the preceding have been made for brick pavements, but it is probable that they are less slippery than asphalt, wood, or stone block. Macadam and gravel are the least slippery of any of the pavements under consideration.

Ease of Cleaning.

The facility with which a pavement may be cleaned is an important matter both economically and esthetically. Col. Geo. E. Waring, noted for his service as Street Cleaning Commissioner of New York city, in 1896 estimated that if all the streets of New York city were paved with asphalt where the grades would permit, the cost of street cleaning would be reduced from $1,200,000 to $700,000 per year. At that time New York had 431 miles of pavement of which 94 were asphalt, and the above annual saving is equal to 3 per cent of the cost of laying asphalt pavements upon all of the streets not already asphalted.

Sheet asphalt pavements are most easily cleaned, and next in order are: asphalt blocks, wood blocks with close joints, brick with joints filled with tar or hydraulic cement, stone block with tar or cement joints, ordinary stone block, round wood block, cobble stone.

Macadam and gravel are smooth and for this reason are easily cleaned; but their surfaces grind up into powder, particu larly under dense or heavy traffic, and for this reason there is considerable detritus to be removed, a fact which adds to the expense of cleaning.

Noiselessness.

The noise made by traffic upon a pave ment has an important effect upon the comfort and health of the people using the pavement or living adjacent to it. A quiet pave

ment is particularly desirable adjacent to office buildings, schools, churches, hospitals, etc.; and the noise of traffic upon a rough pavement aggravates, if it does not cause, nervous disorders.

On sheet asphalt the only noise is the sharp click of the horses' shoes; and on asphalt block there is the click of the feet and a slight rumbling of the wheels over the joints, particularly if the blocks were not laid very close together. Horses' feet make considerable noise on all brick pavements, and wheels produce a decided roar on pavements made of bricks or blocks having rounded corners, at least while the pavements are com paratively new; but if the bricks have square edges and the joints are filled with tar or hydraulic cement, there is only a little rumbling. Stone-block pavements are the most objectionable in this respect, producing a continual roar due both to the rumbling of the wheels and to the blows of the horses' shoes. Upon wood pavements the horses' feet produce no noticeable noise; while the wheels make a dull rumbling noise, but not loud enough to be seriously objectionable. Macadam and gravel are more quiet than wood.

In order of their noise, pavements rank about as follows: stone block, brick, asphalt, wood, gravel, macadam.

Healthfulness.

The effect of a pavement upon the health of the residents in its locality will depend upon the ten dency of the materials composing it to decay and also upon its permeability. Wood is the only paving material that is subject to decay, and is also the only material that is in itself permeable. The gradual decay of the wood is not in itself a serious menace to health; but the decaying wood makes a lodging place for filth and disease germs. The permeability of any wood block is small as compared with that of the wide joints of round wood blocks and of ordinary stone blocks, and it has never been claimed that the ordinary stone-block pavement with its wide and permeable joints was specially unhealthy. It has not been proved that wood pavements appreciably affect the health of a community.

Continuous sheet pavements are best in sanitary qualities, although block pavements having joints filled with tar or hy draulic cement are not seriously objectionable.

Freedom from Dust and Mud.

The materials of an ideal pavement should not grind up and make dust in dry weather or mud in wet weather. The dust and mud not only add to the expense of cleaning the pavement, but are a discom fort to those who use the pavement and to those who live or do business adjacent to it.

Comfort in Use.

If the pavement is to be used for pleasure driving, the comfort of the users must be considered; and therefore the pavement should have a smooth surface which is free from dust when it is dry and from mud when it is wet.

Temperature of Pavements.

During hot weather, there is frequently complaint that one pavement reflects or radiates more heat than another. Observations made in Washington, D. C., when the temperature of the air 2 feet above the pavement was 104° F., showed the temperature of three pavements to be as follows: artificial sheet asphalt 140°, asphalt block 122°, and macadam 118°.* Observations in Boston, when the temperature of the air in the shade was 98° F., gave the temperature of four pavements as follows: wood block 124°, granite block 115°, sheet asphalt 113°, and macadam 102/°. The observations are not conclusive as to the relative temperatures of different pavements, but show that there is no very great difference between the several kinds. The temperature of the pavement depends upon its color, which varies with the material.