【正文】 e of each individual roadway at profile grade while raising the outer edge and lowering the inner edge to produce the desired superelevation is attained some distance beyond the point of curve. If a vehicle travels at high speed on a carefully restricted path made up of tangents connected by sharp circular curve, riding is extremely unfortable. As the car approaches a curve, superelevation begins and the vehicle is tilted inward, but the passenger must remain vertical since there is on centrifugal force requiring pensation. When the vehicle reaches the curve, full centrifugal force develops at once, and pulls the rider outward from his vertical position. To achieve a position of equilibrium he must force his body far inward. As the remaining superelevation takes effect, further adjustment in position is required. This process is repeated in reverse order as the vehicle leaves the curve. When easement curves are introduced, the change in radius from infinity on the tangent to that of the circular curve is effected gradually so that centrifugal force also develops gradually. By careful application of superelevation along the spiral, a smooth and gradual application of centrifugal force can be had and the roughness avoided. Easement curves have been used by the railroads for many years, but their adoption by highway agencies has e only recently. This is understandable. Railroad trains must follow the precise alignment of the tracks, and the disfort described here can be avoided only by adopting easement curves. On the other hand, the motorvehicle operator is free to alter his lateral position on the road and can provide his own easement curves by steering into circular curves gradually. However, this weaving within a traffic lane (but sometimes into other lanes) is dangerous. Properly designed easement curves make weaving unnecessary. It is largely for safety reasons, then, that easement curves have been widely adopted by highway agencies. For the same radius circular curve, the addition of easement curves at the ends changes the location of the curve with relation to its tangents。 does the added width reach 2 ft. For narrower pavements, however, widening assumes importance even on fairly flat curves. Remended amounts of and procedures for curve widening are given in Geometric Design for Highways. B Grades The vertical alignment of the roadway and its effect on the safe and economical operation of the motor vehicle constitute one of the most important features of road design. The vertical alignment, which consists of a series of straight lines connected by vertical parabolic or circular curves, is known as the “grade line.” When the grade line is increasing from the horizontal it is known as a “plus grade,” and when it is decreasing from the horizontal it is known as a “minus grade.” In analyzing grade and grade controls, the designer usually studies the effect of change in grade on the centerline profile. In the establishment of a grade, an ideal situation is one in which the cut is balanced against the fill without a great deal of borrow or an excess of cut to be wasted. All hauls should be downhill if possible and not too long. The grade should follow the general terrain and rise and fall in the direction of the existing drainage