18 HIGHWAY CONSTRUCTION 



road is encumbered with useless ascents, the wasteful expenditure of 

 power is considerable. 



(d) To give the center line such a position, with reference to 

 the natural surface of the ground, that the cost of construction shall 

 be reduced to the smallest possible amount. 



(e) To cross all obstacles (where structures are necessary) as 

 nearly as possible at right angles. The cost of skew structures 

 increases nearly as the square of the secant of the obliquity. 



(/) To cross ridges through the lowest pass which occurs. 



(g) To cross either under or over railroads; for grade crossings 

 mean danger to every user of the highway. 



Examples of Cases to be Treated. In laying out the line 

 of a road, there are three cases which may have to be treated, and 

 each of these is exemplified in the contour map, Fig. 4. First, the 

 two places to be connected, as the towns A and B on the plan, may 

 be both situated in the same valley, and upon the same side of it ; that 

 is, they are not separated from each other by the main stream which 

 drains the valley. This is the si'mplest case. Secondly, although 

 both in the same valley, the two places may be on opposite sides of 

 the valley, as at A and C, being separated by the main river. Thirdly, 

 they may be situated in different valleys, separated by an intervening 

 ridge of ground more or less elevated, as at A and D. In laying out 

 an extensive line of road, it frequently happens that all these cases 

 have to be dealt with. 



The most perfect road is that of which the course is perfectly 

 straight and the surface practically level; and, all other things being 

 the same, the best road is that which answers nearest to this de- 

 scription. 



Now, in the first case, that of the two towns situated on the 

 same side of the main valley, there are two methods which may be 

 pursued in forming a communication between them. A road follow- 

 ing the direct line between them, shown by the thick dotted line A B, 

 may be made, or a line may be adopted which will gradually and 

 equally incline from one town to another, supposing them to be at 

 different levels; or, if they are on the same level, the line should keep 

 at that level throughout its entire course, following all the sinuosities 

 and curves which the irregular formation of the country may render 



