REQUIREMENTS OF AN IDEAL PAVEMENT. The perfect pavement is an ideal which will never be attained, since some of the qualities required in a perfect pavement are antagonistic to each other. For instance, perfect durability would require a pavement without friction, for friction causes wear and ultimately destruction of the pavement; but without friction there could be no adequate foot hold for horses drawing loads. Again, to be the least injurious to horses, a pavement should be soft and yielding; but a soft and yielding pavement is opposed to ease of traction. The conditions to be fulfilled by the ideal pavement will first be considered; and subsequently an attempt will be made to estimate the degree to which each kind of pavement approximates the per fect ideal.
A perfect pavement should satisfy the following conditions: 1. It should be low in first cost.
2. It should be durable, i. e., the cost of perpetually maintain ing its surface in good condition should be small.
3. It should have a smooth, hard surface, so as to have a low tractive resistance.
4. It should afford a good foot hold to enable horses to draw heavy loads, and to prevent them from slipping and falling and possibly injuring themselves and blocking traffic.
5. It should be smooth, so as to be easily cleaned.
6. It should be comparatively noiseless.
7. It should be impervious, so as to keep in good sanitary condition.
8. It should yield neither dust nor mud.
9. It should be comfortable to those who ride over it.
10. It should not absorb heat excessively.
Each of the ordinary forms of pavements will be considered under each of the above requirements.
Cost. The cost of construction of a pavement varies with the character of the work and with the locality. For data on this subject, see the several kinds of pavements in the preceding chap ters.
Durability.The durability of a pavement made of per ishable material, as wood and to some extent macadam, gravel and asphalt, depends upon both the climate and the traffic; but in general the durability of paving materials depends chiefly upon the volume of the traffic, and consequently the durability of differ ent pavements can be accurately compared only when the nature and the amount of the traffic over each is known. Unfortunately
there are very little definite data as to the amount of traffic upon American pavements. Not infrequently the traffic on a particular pavement is referred to as being "heavy" or "light," but such general terms are practically worthless in comparing the durability of different kinds of pavements. Only a few detailed observa tions have been made concerning the traffic upon American pave ments, and they are somewhat antiquated.
"The observations were made during the months of October and November, 1885, by the employees of the Barber Asphalt Paving Company, which has an office and works in the ten large cities. In the arrangements for taking the observations two objects were kept in view: first, to leave as little as possible to the judgment of the observer; and second, to make the record permanent, so that it could be preserved for examination in all its details at any time.
"The agent in each city was instructed to select the three streets in that city paved with stone, asphalt, and wood (if any existed) which, by common report, had the heaviest traffic on the class of pavement used on that street. The record was in every case made on six consecutive days (Sunday excepted) at the same place, and it was continuous from 7 a. m. to 7 p. m., except when darkness prevented. No addition was made for this omission, no record was kept during the night, and no addition was made as an estimate of night traffic.