16 BULLETIN 1077, U. S. DEPARTMENT OE 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 beyond 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 maximum size are assumed, i. e., 
trucks of 204-inch wheel base with a 5-foot gauge, it will be found 
that for curves of 30-foot radius the amount 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 w T idth 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 
