HIGHWAY CONSTRUCTION 



Let A 15, B (\ he two grades in profile, intersecting at station H, 

 and let A and C be the adjacent stations. It is required to join the 



grades by a vertical curve extending from A to C. Imagine a chord 

 drawn from A to C. The elevation of the middle point of the chord 

 will be a mean of the elevations of the grade at A and C, and one- 

 half of the difference between this and the elevation of the grade at 

 B will be the middle ordinate of the curve. Hence we have 



M = 



1 /grade A + grade C 



grade B), 



in which M equals the correction in grade for the point B. The 

 correction for any other point is proportional to the square of its 

 distance from A or C. Thus the correction A H- 25 is ,' ff M; at 

 A + 50 it is J M; at A -f 75 it is T 9 6 M; and the same for corre- 

 sponding points on the other side of B. The corrections in this case 

 shown are subtractive, since M is negative. They are additive 

 when M is positive, and the curve concave upward. 



WIDTH AND TRANSVERSE CONTOUR. 



A road should be wide enough to accommodate the traffic for 

 which it is intended, and should comprise a w r heelway for vehicles 

 and a space on each side for pedestrians. 



The wheelway of country highways need be no wider than is 

 absolutely necessary to accommodate the traffic using it; in many 

 places a track wide enough for a single team is all that is necessary. 

 But the breadth of the land appropriated for highway purposes 

 should be sufficient to provide for all future increase of traffic. The 

 w r heelways of roads in rural sections should be double; that is, one 

 portion paved (preferably the center), and the other left with the 



