The team could probably pull the maximum load up a stretch of the maximum grade 400 to 500 feet long; and if the maximum grade does not occur too often, it could probably pull the load up a stretch two or three times as long. On long maximum grades, it is wise to provide a little stretch of nearly level grade upon which to let the team rest. In the above computation, the team is assumed to have a reserve power equal to that exerted on the maximum grade; but the power required to start the load may be four or five times the normal tractive resistance, and hence a nearly level resting place is required, so that the team may readily start the load.
76. Many of the books on roads state that if the maximum grade is long, the slope should be flattened toward the summit to com pensate for the decreased strength of the fatigued horses. This recommendation is both improper and impracticable. It is improper, since it assumes that if the horse is to develop energy to lift the load up the incline, he should not work at a uniform rate. Universally the race horse goes fastest on the home stretch; and if he is urged to his utmost speed at first, he is sure to lose the race. The recommendation is impracticable, since the topography would rarely permit the flattening of the grade at the top without in creased expense, and it would not be wise to incur extra cost for this purpose.
If the loads are much heavier in one direction than in the other, it is permissible to oppose the lighter traffic with the steeper ruling grade.
As a Descent. Viewed as a descent, the maximum grade concerns chiefly the safety of rapid traveling. Many of the writers on roads claim that the descending grade should not exceed the angle of repose, i. e., should not exceed the inclination down which the vehicle will descend by its own weight. This limit is impracti cable, since the angle of repose varies with the kind of vehicle, degree of lubrication, amount of load, size of wheels, etc. Besides, this limitation is unnecessary, since the resistance of traction in creases as the speed, and in going down it is only necessary to drive faster to prevent the vehicle from unduly crowding upon the team; but of course this remedy has its limitations. Further, the speed in descending may be checked by the application of the brake; but it should be remembered that the use of the brake is detrimental to the road surface, particularly on the maximum grade.
On ordinary roads, grades twice as steep as the angle of repose are operated without inconvenience or danger. In Europe it is usually assumed that on a good broken-stone road, of which the angle of repose is about 2 or 21 per cent, a 5 per cent grade is the maximum that can be descended safely at a trot without brakes; and, if the stretch is long, 3 per cent is considered the maximum for safety. On mountain roads having a broken-stone surface, freight wagons descend 12 per cent grades by the use of brakes, but with expert drivers.
Examples of Maximum Grades for Earth Roads. For ob vious reasons there are not much data under this head. In hilly country short grades of 1 in 3 (33%) are occasionally found—par ticularly in a new country,—and grades of 1 in 4 (257:) are some what common. In comparatively flat country. grades of 1 in 8 (14%) are not infrequent.
In improving the celebrated Holyhead road, Telford found in old roads many grades of 1 in 6 and 1 in 7. A number of roads improved by state aid in New Jersey originally had grades of 14 per cent. Of course, only the roads having the most traffic were improved; and less frequented roads in the same locality have much greater grades.
Examples of Maximum Grades on Broken-stone Roads. In Prussia the standard is: in mountainous country 1 in 20 (5%), in hilly country 1 in 25 (4%), and in level country 1 in 40 (2i%).
In Hanover the regulations are: in mountainous country 1 in 24 (4%), in hilly country 1 in 30 (4%), and in level country 1 in 40 (4%).
In Baden the standard is: main highways 5 per cent, secondary roadways 6 per cent, and mountain roads 8 per cent.
In Brunswick the regulations are: on the plains 1 in 331 (3%). in hilly country 1 in 25 (4%), and in mountainous country 1 in 18 In France the standard is: on national roads, not exceeding 3 per cent; departmental roads, not exceeding 4 per cent; and subordinate roads, not exceeding 6 per cent. On the great Alpine road over the Simplon Pass, built under the direction of Napoleon Bonaparte, the grades average 1 in 22 (4i%) on the Italian side, and 1 in 17 (5.9% ) on the Swiss side, and in only one case become as steep as 1 in 13 (7.7%).