ROADS AND PAVEMENTS 



393 



frequently becomes the controlling factor, but it is not always 

 wise to cut down initial cost to the lowest amount possible; 

 such apparent economy may result in the construction of a road 

 requiring for its maintenance much trouble and expense, which 

 might have been avoided by a small extra cost in the original 

 construction. A better plan, and one that should always be 

 followed, is to arrange the road so that future improvements 

 can be made. 



Minimum and Maximum Grades. In order that efficient 

 drainage may be provided for the roadway, the minimum grade 

 should generally not be natter than 1%, and should never be 

 materially natter than one-half of 1%, except on first-class 

 pavements. In general, the maximum grade should not be 

 steeper than 9% for earth roads, 6i% for gravel roads, and 3% 

 for macadam roads, in any case where it is possible to keep 

 within these limits; and, preferably, should never be steeper 

 than about 3 to 5% for any kind of road. 



As a result of investigations, it has been deduced that, depend- 

 ent on the amount of traffic and the cost of construction and 

 maintenance of the road, the most advantageous gradients vary 

 for mountainous country between 5 and 3% ; for hilly country, 

 between 3 and 2J%; and for gently rolling country, between 

 2J and 1%. 



Form of Cross-Section. One of the best forms for highways 

 is a parabolic arc, as is shown in the accompanying illustration. 

 Its construction is as follows: 



Divide the width AB between the edge of the gutter and 

 the center of the wheel way into ten equal parts, and at the 



