HIGHWAY CONSTRUCTION 



of the convexity. Circular arcs, two straight lines joined by a circular 

 arc, and ellipses, all have their advocates. 



TABLE 10. 



Kind of Surface. Proportions of tho 



Carriageway. Width. 



Earth Rise at center ? > 



Gravel ^ 



Broken Stone -J, t 



For country roads a curve of suitable convexity may be obtained 

 as follows: (jive I of the total rise at \ the width from the center 

 to the side, and - of the total rise at \ the width (Fig. 9). 



Excessive height apd convexity of cross-section contract the 

 width of the wheel way, by concentrating the traffic at the center, 

 that being the only part where a vehicle can run upright. The force 

 required to haul vehicles over such cross-sections is increased, be- 



,^^^^ 



Fig. 9. 



cause an undue proportion of the load is thrown upon two wheels 

 instead of being distributed equally over the four. The continual 

 tread of horses' feet in one track soon forms a depression which holds 

 water, and the surface is not so dry as with a flat section, which allows 

 the traffic to distribute itself over the whole width. 



Sides formed of straight lines are also objectionable. They 

 wear hollow, retain water, and defeat the object sought by raising 

 the center. 



The required convexity should be obtained by rounding the 

 formation surface, and not by diminishing the thickness of the 

 covering at the sides. 



Although on hillside and mountain roads it is generally recom- 

 mended that the surface should consist of a single slope inclining 

 inwards, there is no reason for or advantage gained by this method. 

 The form best adapted to these roads is the same as for a road under 

 ordinary conditions. 



With a roadway raised in the center and the rain water draining 

 off to gutters on each side, the drainage will be more effectual and 



