RATE OF GRADE.
The effect of any change in the ruling gradient upon a road depends to a considerable extent upon what portion of the traffic may be carried in full loads. The lighter portions of the traffic are not so seriously affected by heavy gradients as the heavy portions, although there is an advantage in light gradients for any driving. The rate of speed which may be employed will be less upon the portions of the road having heavy grade, and the time occupied in a trip over the road is therefore affected somewhat by the rate of grade.
The desirability of a road for general driving is also much influenced by the gradients employed, as is that value of the road which has for a basis the effect it may exert upon the attractiveness of the locality. These things all have a certain financial value, which of course it is quite impossible to estimate with any degree of accuracy, but which should be considered in determining the allowable maximum gradient in any case in practice.
For heavy traffic, such as the transfer of goods from one town to another or the marketing of country prod uce, the limitation of load placed upon the traffic by the gradient is a matter of importance, the effect of which is calculable upon the cost of transportation. If in any case the approximate amount of heavy traffic which is likely to be carried in full loads be' deter mined, the relative costs of its transportation over two lines of differing gradient, other conditions being similar, will be nearly proportional to the number of loads required to move traffic over each gradient.
In estimating the value of reducing the rate of grade, it may be considered, as in the case of a reduction of length, that its value to the community is represented by the saving in annual costs of transportation, and that the amount that may reasonably be expended in increased cost of construction to effect a reduction of gradient is the sum upon which this annual saving is the interest.
The length of a road and the amount of rise and fall on it determine the amount of work that must be done in hauling a load over the road. The rate of gradient, on the contrary, does not affect the amount of work necessary to move the traffic, but it limits the work that a horse may do at one trip.
The establishment of a proper rate for the ruling grade of the line is, therefore, usually the most impor tant point in location. In localities where light gra dients are easily obtained the problem of location is greatly simplified.
By referring to Art. 3 the comparative loads that a horse may draw up different grades will give some idea of the importance of carefully considering the question of gradient. In nearly all cases in practice there is a considerable latitude within which gradients may be chosen. It is usually a question of heavier gradients as against greater distance and larger first cost for the road. It may be remarked that it is only under exceptional circumstances that it is either neces sary or advisable to use a steeper gradient than 5 per cent on the new location of a country road of any importance. Grades steeper than the ruling gradient may sometimes be introduced over short distances without impairing the efficiency of the road, as horses are usually able to exert for a short time a force much greater than they can continuously exert. If the length of grade be quite short, 200 or 300 feet, a horse can about double his ordinary power in passing it.
Where long steep grades must be used, it is desirable to break them by short stretches of lighter gradients to provide resting-places for horses.
Heavy gradients also have the disadvantage of retarding traffic in the direction of falling grade, and, as suggested in Art. i8, of requiring the expenditure of work to hold the load from too rapid descent.