74 



HIGHWAY CONSTRUCTION 



which fancy dictates. Whatever width is adopted for them, not more 

 of it than 8 feet need be paved, the remainder being occupied with 

 grass and trees. 



Street Grades. The grades of city streets depend upon the 

 topography of the site. The necessity of avoiding deep cuttings or 

 high embankments which would seriously affect the value of adjoining 

 property for building purposes, often demands steeper grades than 

 are permissible on country roads. Many cities have paved streets 

 on 20 per cent grades. In establishing grades through unimproved 

 property, they may usually be laid with reference to securing the most 

 desirable percentage within a proper limit of cost. But when improve- 

 ments have already been made and have been located with reference 

 to the natural surface of the ground, giving a desirable grade is fre- 

 quently a matter of extreme difficulty without injury to adjoining 

 property. In such cases it becomes a question of how far individual 



interests shall be sacrificed to the 

 general good. There are, how- 

 ever, certain conditions which it is 

 important to bear in mind: 



(1) That the longitudinal 

 crown level should be uniformly 

 sustained from street to street 

 intersection, whenever practicable. 



(2) That the grade should 

 be sufficient to drain the surface. 



(3) That the crown levels at 

 all intersections should be ex- 

 Fig- 47 - tended transversely, to avoid form- 

 ing a depression at the junction. 



Arrangements of Grades at Street Intersections, The best ar- 

 rangement for intersections of streets when either or both have much 

 inclination, is a matter requiring much consideration, and is one upon 

 which much diversity of opinion exists. No hard or fast rule can be 

 laid down; each will require special adjustment. The best and sim- 

 plest method is to make the rectangular space aaaaaaaa, Fig. 47, 

 level, with a rise of one-half inch in 10 feet from AAAA to B, placing 

 gulleys at AAAA and the catch basins at ccc. When this method" is 

 not practicable, adopt such a grade (but one not exceeding 2 J per cent) 



a 



