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Comparison of Routes

line, gradient, traffic, road, considered, lines, valley, length and importance


In selecting a line for the construction of a road the principles already mentioned in the early part of this chapter should be had in mind. The line must be well designed to accommodate the traffic. It should have as easy grades, short length, and small rise and fall as is consistent with a reasonable cost of construction, in order to give light costs for transportation and for maintenance.

Suppose in the case shown in Fig. 11 that it is desired to connect the village at the point A with the point D and with the roads leading through the passes at F and I. Which line it will be the most advantageous to adopt depends upon the relative importance of the traffic to the various points considered.

The shortest, and probably cheapest, line from A to D would be obtained by following the valley over the line ABCD, which line, as shown by the profile, would give a maximum gradient of Jo in ioo between C and D. The line FB joining the first line at B would afford communication with the summit at F with a maximum gradient of 5 in ioo. If the traffic to the point D be small and unimportant, so that addi tional expense in reducing the gradient from C to D is unadvisable, these lines might prove a satisfactory location.

If, however, D be a point of importance and the traffic from A to D heavy, it will be necessary to adopt some means to reduce the gradient from C to D. Leaving out of consideration the point F and consider ing B and C as points of minor importance, it might be advisable to use the line ALMD with a uniform 5 per cent gradient from D to L, and branches to connect with C and B. This would give a line but little longer than the valley line, with only one-half the ruling gradient of that line.

If C is not important and can be neglected while B and F must be considered, the line ABEHD has a maximum gradient of 5 in zoo, and connects A with the points BF and D with a minimum total length of road (being less than the valley line first considered).

When B and C must both be considered as of im portance as well as F and D, the lines ABCHE and HD will give a ruling gradient of 5 in zoo to both F and D, and passing through B and C with a somewhat longer line than in the last case.

This arrangement would make the length of haul from A to D and F each longer than by the first line considered; but the gradient to D would be lighter, and the total length of road to be constructed and main tained would be less.

In case the points B and C are both unimportant, and the line through the valley may be neglected, the line AGFD provides a ruling gradient of 4 in no from A to both F and D, and connects them with each other, with about the same length as the shortest 5 per cent gradient. When the point I must be taken into account, this line may be connected with I by the line GI having a gradient of 4 in zoo. This would give the shortest line of uniform gradient to connect A with the three points I, F and D, and possibly a desirable line to construct when the line through the point I is impor tant, even if the valley road from A to B is also neces sary.

The lines upon the side slopes are usually more ex pensive to construct than the valley lines, and the dif ferences of first cost of the various lines must of course be considered. The importance of a difference in ex pense of construction depends upon the traffic to be hauled over the road and the kind of surface to be used. Where a broken-stone or gravel road is to be constructed at considerable expense, the difference of cost due to a change of location is relatively less important as being a less percentage of the whole cost, while the difference of tractive effort due to grade is of more importance, as being a higher percentage of that upon the level, than would be the case with an ordinary earth road.

As is easily seen from the above the choice of location for a road, while depending upon principles easily stated, is in reality a matter requiring the use of judgment, and is not readily reducible to a financial comparison stated in money values, because the data concerning the volume of the traffic and the cost of conducting it can be determined only very roughly, and contains many elements of error. For purposes of comparison to aid the judgment, approximate data may often be assumed or determined by a study of the localities affected. In some cases observations may be made of the number of teams of different classes pass ing certain points within certain times, to give a basis for estimation of the annual volume of traffic. In other cases, the annual hauling traffic, which is usually the most important portion of the traffic in considering location, may be estimated from the known interests of the locality. Thus, if the produce of a certain section of farming country must be hauled over a given road to market, the amount of this produce may be esti mated from the acreage, and the relative number of loads upon different , grades then determined. The cost per load over the road would then need to be as sumed in order to find the annual value of a reduction of grade.

In the same manner, the effect of changes of length and in the amount of rise and fall may be found as indicated in Arts. 17 and i8.

All of these items must be combined to find the rela tive total costs of transportation for each route. The cost of construction and of maintenance for each line must then be estimated, and that line is the most ad vantageous which makes the sum of the annual charges and the interest on the first cost a minimum. Where several lines of traffic are to be considered together as in Fig. it, the cost of conducting all of the traffic by each system of lines that may be employed must be considered, the entire cost being made a minimum for the system to be adopted.