EARTH, SAND-CLAY, AND GRAVEL ROADS. 11 



WIDTH. 



The minimum width to accommodate safely two lines of average 

 horse-drawn traffic is 14 feet, and for automobile traffic the width 

 preferably should be not less than 18 feet, though a width of 1G feet 

 is used frequently. In order to maintain the traveled way to the re- 

 quired width and to afford proper safeguards against accidents, it 

 is necessary to provide a shoulder not less than 3 or 4 feet wide along 

 each side of the roadway proper. The shoulders may have a some- 

 what steeper crown than the rest of the road surface, as shown on the 

 cross-section drawings, figures 21 and 22, but they should be suffi- 

 ciently flat not to endanger traffic using them and really should con- 

 stitute an additional width of roadway. This means that the total 

 width of roadway between side ditches never should be less than 20 

 feet where horse-drawn traffic predominates, and 24 feet where any 

 considerable volume of automobile traffic is to be accommodated. 



Where sharp curves occur in the alignment it is desirable, though 

 not customary, to increase the width of the traveled way. A vehicle 

 being drawn along a curved road tends to occupy an appreciably 

 greater width than where the road is straight, and unless the width 



Slope i to f per foot, depending on the rate of curvature and grade 



CROSS SECTION SOMETIMES USED ON SIDE HILL CURVES TO PREVENT 

 SKIDDING OF VEHICLES AND EROSION OF THE SLOPES. 



Fig. 9. 



of the traveled way is increased correspondingly, this tendency con- 

 tributes materially to the hazards that invariably accompany sharp 

 curves. The minimum widths given above should also be increased 

 on embankments of any considerable depth, so as to make maintenance 

 easier and at the same time diminish the danger of accidents. 



The width of right of way required to provide all necessary area 

 for the roadway, slopes and ditches varies considerably with the 

 nature of the topography. Where the topography is flat, or gently 

 rolling, a 50-foot right of way usually will be sufficient for a 20-foot 

 roadway. But where the country is very rolling or hilly the width 

 of right of way usually should be at least 60 feet. For roadways 

 wider than 20 feet it is usually sufficient to increase the width of 

 right of way by the same amount that the width of roadway is 

 increased. 



GRADES. 1 



In designing a public road one of the most difficult problems to 

 solve properly is the question of maximum allowable grades. In 



1 E. B. McCormick, mechanical engineer, U. S. Office of Public Roads and Rural. Engi- 

 neering, has furnished the data on which this discussion is based. 



