HIGHWAY CONSTRUCTION 23 



Alignment. No rule can he laid down for the alignment of a 

 road; it will depend both upon the character of the traffic on it and 

 upon the "lay of the land." To promote; economy of transportation 

 it should be straight; but if straightncss is obtained at the expense 

 of easy grades that might have been obtained by deflections and 

 increase in length, it will prove very expensive to the community 

 that uses it. 



Where curves are necessary, employ the greatest radius possible 

 and never less than fifty feet. They may be circular or parabolic. 

 The parabolic will be found exceedingly useful for joining tangents 

 of unequal length, and for following contour lines; its curvature 

 being least at its beginning and ending, makes the deviations from 

 a straight line less strongly marked than by a circular arc. 



When a curve occurs on an ascent, the grade at that place must 

 be diminished in order to compensate for the additional resistance of 

 the curve. 



The width of the wheelway on curves must be increased. This 

 increase should be one-quarter of the width for central angles between 

 90 and 120 degrees, and one-half for angles between 60 and 90 degrees. 

 Excessive crookedness of alignment is to be avoided, for any unneces- 

 sary length causes a constant threefold waste; first, of the interest 

 of the capital expended in making that unnecessary portion; secondly, 

 of the ever recurring expense of repairing it; and thirdly, of the time 

 and labor employed on travelling over it. 



The curving road around a hill may be often no longer than the 

 straight one over it, for the latter is straight only with reference to 

 the horizontal plane, while it is curved as to the vertical plane; the 

 former is curved as to the horizontal plane, but straight as to the 

 vertical plane. Both lines curve, and we call the one passing over 

 the hill straight only because its vertical curvature is less apparent 

 to our eyes. 



The difference in length between a straight road and one which 

 is slightly curved is very small. If a road between two places ten 

 miles apart were made to curve so that the eye could nowhere see 

 farther than one-quarter of a mile of it at once, its length would 

 exceed that of a straight road between the same points by only about 

 four hundred and fifty feet. 



