ROADS AND PAVEMENTS 409 



tion of both. The pavement is constructed in much the same 

 manner as an asphalt pavement. The foundation is com- 

 posed of a 4-in. layer of broken stone compacted by rolling. 

 The interstices are filled and the surface is covered with 

 bituminous cement. The material for the wearing surface 

 is heated to about 250 F., spread while hot, and compacted 

 by rolling with a 10- T. roller to a thickness of about 2 in. 

 The surface is then covered with a liquid bituminous cement, 

 on which, while it is in a sticky condition, there is spread a 

 layer of sand or stone dust to a depth of about | in. The 

 rolling is then repeated, after which the pavement is ready 

 for use. 



CITY STREETS 



Width. The roadway of a city street should be of such a 

 width as to accommodate the traffic. For business streets, a 

 width of roadway from 40 to 80 ft. is required, and for residence 

 streets it should generally be from 24 to 36 ft. The sidewalks 

 on business thoroughfares usually extend from the curbing to 

 the building line, and on residence streets the width is about 

 one-fifth to one-sixth the width of the roadway. The outer 

 edges of the sidewalks on residence streets are commonly placed 

 about 2 ft. from the fence line. 



Height of Crown. Let w be the width of the roadway, in 

 feet; /, the per cent, of grade; and q, a coefficient given in the 

 table on page 411. Then the height of crown in feet is 

 wp(70q- 1) 



FIG. 1 



When the grade is comparatively level, the height of crown is 

 determined in the same manner as for highways, previously 

 given. Expressed by a formula 



c = qw 



