18 BULLETIN 107-7, U. S. DEPARTMENT OF AGRICULTURE. 
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 upon the greater clearance 
required on curves for additional safety. At what point widening 
should be discontinued is someAvhat 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 from 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 curA^e so small that a true transition curve will not 
satisfy the conditions. For curves of 200-foot radius and OA r er the 
transition curA^e Avill give satisfactory results. 
If it is desired to use transition curves to connect the circular 
curve and the tangents, Avidening may be accomplished as shoAvn in 
