IRRIGATION PRACTICE AND HIGHWAYS 331 



on center lines of fifty feet and slopes flattened and 

 cleared to afford clear sight of at least one hundred 

 and fifty feet. Permanent culverts, gutters and 

 ditches are provided when needed to prevent water 

 from standing on the roadsides. Bridges are of re- 

 inforced concrete, twenty-one feet wide in the clear 

 and designed to carry sixteen-ton tractor engines. A 

 minimum width of roadway of sixteen feet is re- 

 quired in the mountains ; an average width elsewhere 

 through the State of twenty-four feet, twenty-one 

 feet through cuts and twenty-two and one-half feet 

 where the road is part cut and part fill. 



After grading and rolling, a cement pavement was 

 laid, for which the following specifications were 

 chiefly used : The concrete base was generally fifteen 

 feet in width and of prevailing thickness of four 

 inches. When necessary the roadway was often wid- 

 ened by three-foot macadam or gravel shoulders on 

 each side of the pavement. Upon the concrete pave- 

 ment a bituminous wearing surface was placed, con- 

 sisting of half an inch in thickness of heavy asphaltic 

 road oil and screenings, which has been very satisfac- 

 tory in protecting the concrete pavement from wear. 



It should be noted that there is an intimate rela- 

 tion between climatic conditions and the success of 

 the highway system which has been for years ex- 

 tending to greater mileage. No state which has a 

 wintry climate can safely use such specifications. As 

 the California Highway Engineer remarks: "It 

 must be admitted that a concrete base as thin as 

 four inches would not be at all suitable in localities 



