16 BULLETIN 1077, u. s. depaetme]s:t of agriculture. 



the lowering of the inner edge, the superelevation may be obtained 

 by rotating the pavement about the inner edge, i. e., by raising the 

 outside of the pavement. The maximum superelevation should be 

 obtained at the point of curve and continued for the entire length 

 of the curve. The pavement should begin to gain superelevation at 

 a point on the tangent approximately 100 feet from the beginning of 

 the curve, reach a maximum at the point of curve, and ease off to the 

 regular pavement cross section the same distance bej^ond the point 

 of tangency. 



WIDENING ON CURVES. 



In rounding a curve the rear wheels of a vehicle travel on a shorter 

 radius than the front wheels. On this account a greater width 

 of pavement is occupied by the vehicle on curves than on tangents. 

 The additional width varies with the radius of the curve, the gauge 

 of the wheels and the length of the vehicle. To allow the same 

 clearance between passing vehicles on curves as on tangents the 

 width of the pavement on the curves should be increased by an 

 amount equal to the sum of the additional widths required by the 

 two vehicles. If two vehicles of maximimi size are assumed, i. e., 

 trucks of 201:-inch wheel base with a 5-foot gauge, it will be found 

 that for curves of 30- foot radius the amoimt of widening required 

 is 12.5 feet, while for curves of 150-foot radius the additional width 

 is 2 feet and for a radius of 500 feet, only 0.5 foot. For curves of 

 more than 500-feet radius the additional width required is negligi- 

 ble. 



If the passing vehicles are two automobiles of averge size instead 

 of two large trucks the additional width required will be less 

 on account of the shorter wheelbase and narrower gauge of the 

 smaller vehicles. If provision is to be made for the passage of 

 a truck and an automobile the extra width required will be between 

 the larger and the smaller amount. In widening curves the added 

 width should be consistent with the provision that has been made 

 on tangents. If the normal section on tangents is 16 feet wide the 

 road will accommodate two automobiles in passing and the addi- 

 tional width on curves should be designed to provide for two such 

 vehicles. The 18-foot normal section provides for the passage of 

 an automobile and a truck, and the 20-foot section accommodates 

 two large trucks. The additional width on curves, therefore, should 

 provide for the passage of vehicles of the same type. The method 

 of computing the amount of widening required is illustrated in 

 Figure 3. 



Theoretically the amount of widening determined in this manner 

 is all that is required, but an additional allowance of a foot or two 



