18 BULLETIN 1077, r. S. DEPAETMEXT OF AGEICL^LTURE. 



The table on page 19 gives the amount of -widening for curves 

 of 16, 18, and 20 foot pavements, computed on the basis of the 

 above assumptions for curves up to 500-foot radius. For curves of 

 greater radius than 500 feet the amount of widening -would be 

 practically constant and would be based uj)on the greater clearance 

 required on curves for additional safety. At what point widening 

 should be discontinued is somewhat problematical, but it is believed 

 no additional clearance in passing is required for curves of 1,000 

 feet radius or greater. 



It is now generally agreed that the increased width should be added 

 to the inside rather than the outside of the curve ; but there is con- 

 siderable difference of opinion as to where the widening should 

 begin. If the path of a vehicle around a circular curve is analyzed, 

 it will be found that as the front wheels conform to the curve, the 

 rear wheels effect a gradual transition to concentric curves of shorter 

 radii and then follow these concentric curves around the circle. This 

 transition of the rear wheels to curves of shorter radii begins on the 

 tangent approximately one vehicle length from the point of curve 

 and is generally completed in fi'om one to one and one-half vehicle 

 lengths on the curve. The necessity for curve widening, therefore, 

 exists practically for the entire length of the circular curve; and 

 for curves ordinarily used in highway practice, full widening should 

 obtain both at the point of curve and the point of tangency. 



The logical method of widening curves, therefore, is on the inside, 

 full amount of widening for the entire length of circular curve. To 

 gain this width at the two ends of the circular curve it is necessary 

 that the widening of the pavement he begun at some distance from 

 the points of curvature and tangency, thus providing a widening 

 approach section to the curve. Theoretically, the length of this ap- 

 proach section should be varied with the degree of the curve, but in 

 practice it is customary to employ a uniform length for all curves. 

 A simple design which has proved satisfactory is shown in Figure 4, 

 in which the approach section is in the form of a taper and the 

 widening is begun at a distance of 100 feet from the ends of the 

 circular curve. Instead of a straight-line taper, a transition curve 

 may be used. In this case the offset from the tangent to the circular 

 curve would be equal to the amount of widening and would deter- 

 mine the length of transition curve which would have to be used. 

 A transition curve, however, cannot be used on widened curves of 

 very short radii, because the amount of widening is so great and the 

 length of circular curve so small that a true transition curve will not 

 satisfy the conditions. For curves of 200-foot radius and over the 

 transition curve will give satisfactory results. 



If it is desired to use transition curves to connect the circular 

 curve and the tangents, widening may be accomplished as shown in 



