4.7.6 Superelevation Transition Length

Superelevation transition is the general term denoting the change in cross slope from a normal crown section to the full superelevated section or vice versa. To meet the requirements of comfort and safety, the superelevation transition should occur over a length adequate for the usual travel speeds.
Transition lengths should also account for potential future traveled way widening, including widening associated with the ultimate typical section in a schematic.
Preferable design values for length of superelevation transition are based on a given maximum relative gradient between profiles of the edge of traveled way and the axis of rotation.
Table 4-8
shows recommended maximum relative gradient values. Transition length on this basis is directly proportional to the total superelevation, which is the product of the lane width and the change in cross slope.
Table 4-8: Maximum Relative Gradient (G) for Superelevation Transition
Design Speed
(mph)
Maximum
Relative Gradient
1
(%)
Equivalent Maximum
Relative Slope (run:rise)
15
0.89
1:112
20
0.80
1:125
25
0.73
1:137
30
0.67
1:150
35
0.62
1:162
40
0.57
1:175
45
0.53
1:187
≥50
0.50
1:200
Notes:
  1. Maximum relative gradient for profile between edge of traveled way and axis of rotation.
Preferable transition length, L
CT
, can be calculated using the following equation:
LCT(preferable)=CSWG
Where:
L
CT (preferable)
 =
Calculated preferable transition length, ft
CS =
 Change in cross slope of superelevated pavement, percent
W =
 distance between the axis of rotation and the edge of traveled way, ft
G =
 maximum relative gradient (%).
Example determinations of superelevation transition are shown in
Figure 4-2
.
Determination of Length of Superelevation Transition ( click in image to see full-size image)Determination of Length of Superelevation Transition Example no. 1( click in image to see full-size image)Determination of Length of Superelevation Transition Example no. 2 ( click in image to see full-size image)
Figure 4-2: Determination of Length of Superelevation Transition
As the number of lanes to be transitioned increases, the length of superelevation transition increases proportionately with the increased width. While strict adherence to the length (L
CT
) calculation is preferable, the length for multilane facilities may become impractical for design purposes (e.g., drainage problems, avoiding bridges, accommodating merge/diverge condition).
A minimum length (L
CT
), can be calculated using adjustment factors as shown in
Table 4-9
, such that the transition length formula becomes:
LCT(min)=CSWGb
where “b” is defined in Table 4-9.
In the case of one lane being rotated, “b” is 1.0, such that L
CT (min)
 = L
CT (preferable)
Table 4-9: Multilane Adjustment Factor
1
Number of Lanes Rotated
(n)
Adjustment Factor
2
(b)
1.5
0.83
2
0.75
2.5
0.70
3
0.67
3.5
0.64
4
0.63
4.5
0.61
5
0.60
Notes:
  1. . These adjustment factors are directly applicable to undivided facilities. For divided facilities where the axis of rotation is not the edge of traveled way, see AASHTO’s A Policy on Geometric Design of Highways and Streets.
  2. For all values of n, b = 1+0.5(n−1) / n (Calculations are rounded to the nearest hundredth)