Unfortunately it is not possible to determine the amount of traffic with any considerable degree of accuracy. At some railroad stations the sole freight shipped out is agricultural produce, in which case the traffic over any particular wagon road can be approx imated by distributing the shipments according to the contributing area. The average load can be determined with sufficient accuracy by consulting the records of the grain dealers. In addition to the above, which may be called the heavy freight traffic, there is a considerable amount of light freight and passenger traffic which would be benefited by a saving of distance.
For the sake of working out an example, it will be assumed that the cost of transportation is 10 cents per ton-mile. This cost is made up of the cost of loading and unloading, of driving, of feed, and of wear and tear of horses, wagon, and harness. The cost of loading and unloading is independent of distance. The cost of driving nominally varies as the time, i. e., as the distance (see third paragraph of § 60). The cost of feed and of wear and tear varies as the distance. It is impossible to assign reliable values to these several factors of the cost, but it is certain that only part of the cost of transportation varies as the distance; and for the sake of com pleting the illustration, it will be assumed that 8 cents per ton-mile varies as the distance. This sum multiplied by the number of tons passing over the road in a year will give the sum that may be spent annually to secure a saving of 1 mile of distance.
For example, a road leading to a certain village was originally laid out on the east and north sides of a quarter-section, but on account of low ground on the northeast corner another road was opened on the south and west sides. The quarter-section was one large field. How much expense would the traffic justify in order to secure a road diagonally through the quarter-section? The heavy freight traffic was approximately 3.000 loads of 1 ton each per annum.
The annual value of saving 1 mile would then be 8 cents X3,000= $240. The saving in distance by going through the quarter-sec tion is 0.29 mile; and the annual value of saving this distance is 4240X 0.29 = $69.60. The diagonal road occupies 2/ acres less land than the longer one; and as the land rented for $3 per acre, this adds $3 X 2/ = $7 per annum to the value of the diagonal road. The annual saving from these two items is then $69.60 + $7.00= $76.60. This is the interest at 5 per cent on $1,532, which sum, according to the above computations, could be borrowed, and used to secure this improvement, and the community be no worse off financially.
In addition, there would be some advantage to the light freight and passenger traffic by shortening the road, but it is difficult, if not impossible, to estimate this saving; and as the benefit per trip would probably be less than for the heavy freight traffic, it was neglected. There would be a slight saving in the cost of mainte nance of the shorter road, as in this case the soil and drainage was as good on one line as on the other. Further, there would be some saving on the return trip by the shorter road. On the other hand, it is probable that the smaller number of acres required for the diagonal road would cost at least as much as for the road around the quarter-section, owing to the farmers' justifiable dislike for non rectangular fields, and because the diagonal road would divide the quarter-section.
There are several matters that materially affect the relia bility of the method of the above investigation. In the first place, the cost of transportation can not be known with any degree of reliability. The farmers concerned would stoutly contend that the price assumed above is much too great (see § 20-21) ; while freighters would claim that the value assumed was too low (see § 4-7).
In the second place, not all of the computed annual saving is available for making the improvement, since some of it should be set aside to form a sinking fund to be used ultimately in extinguish ing the debt. It is not the part of wisdom to extend the debt very far into the future, since the conditions may materially change. For example, a new railroad may divert the traffic from this par ticular road, or improvements in the condition of the surface of the road may decrease the cost of transportation,—either of which would decrease the value of the proposed improvement. Of course, certain contingencies may increase the traffic and thereby add to the value of the improvement; but it is not wise to incur a definite debt for an equal and somewhat problematic saving. Road re formers sometimes overlook the fact that interest is a yearly charge and that the debt must finally be paid.