410 ROADS AND PAVEMENTS 



Form of Crown. For laying out a curving crown, Fig. 1, 

 the method given under Highways may be used, or the following 

 formula may be employed: 



in which x and y are, respectively, the abscissa and ordinate to 

 any point p in the surface line of the cross-section with reference 

 to the origin o. 



FIG. 2 



For a sloping crown. Fig. 2, the portions tg and t'g' have a 

 uniform slope of 



4c 



S ~2w-b' 



in which b is the width of the parabolic portion It'. This para- 

 bolic portion may be constructed by the formula 



in which x c and y c are the coordinates of any point with reference 



sb 

 to o as an origin. The ordinate at the tangent point t is y t = , 



and the coordinates to any point P along the straight slope line 

 t g are related by the formula 



H 



Grades. In order that the surface water may be promptly 

 and effectually removed from a roadway, the rate of grade for 

 the street should never be less than one-fourth of 1%, that is, 

 .25ft. per 100ft.; the grade should not be as flat as this except 

 in extreme cases and with first-class pavements, such as brick 

 or asphalt. A minimum grade of one-half of 1%, is as flat as 

 should generally be used, and a grade as steep as 1% is very 



