Chapter 4 - Elements of Design
4.5. Superelevation
4.5.4. Superelevation Transitions
Development of Superelevation
For appearance and comfort, the length of superelevation runoff (and tangent runout) should be based on a relative gradient between the longitudinal grades of the axis of rotation and the outside edge of travelway pavement. The maximum relative gradient is varied with design speed to provide longer runoff lengths at higher speeds and shorter lengths at lower speeds. The maximum relative gradients are depicted in Table 4.11. Maximum Relative Gradients. These values correspond to those found in the AASHTO Green Book.
Table 4.11. Maximum Relative Gradients |
Design Speed (mph) |
Maximum Relative Gradient, G (%) |
Equivalent Maximum Relative Slope |
14
20
25
30
35
40
45
50
55
60
65
70
75
80 |
0.78
0.74
0.70
0.66
0.62
0.58
0.54
0.50
0.47
0.45
0.43
0.40
0.38
0.35 |
1:128
1:135
1:143
1:152
1:161
1:172
1:185
1:200
1:213
1:222
1:233
1:250
1:263
1:286 |
Refer to the AASHTOGreen Book for guidance on establishing superelevation runoff lengths, superelevation (tangent) runout lengths and locating superelevation transitions.
AASHTO guidelines shall be followed when determining and locating superelevation runoff, runout and transitions. When AASHTO values cannot be attained for superelevation parameters, a design exception or design variance is required.
Table 4.11. Maximum Relative Gradients indicates that relative gradients vary per design speed. A strict application of the maximum relative gradient criterion provides runoff lengths for four-lane undivided roadways that are double those for two-lane roadways; and those for six-lane undivided roadways would be tripled. While lengths of this order may be desirable, it is often not practical to provide such lengths in design. It is recommended that minimum superelevation runoff lengths be adjusted downward to avoid excessive lengths for multilane highways.
The recommended adjustment factors are presented in Table 4.12. Adjustment Factor for Number of Rotated Lanes. These values correspond with the values found in the AASHTO Green Book (2004).
Table 4.12. Adjustment Factor for Number of Rotated Lanes |
Number of Lanes Rotated (N1) |
Adjustment Factor
(bw) |
Length Increase Relative to 1 Lane Rotated (=N1bw) |
1.00
1.50
2.00
2.50
3.00
3.50 |
1.00
0.83
0.75
0.70
0.67
0.64 |
1.00
1.25
1.50
1.75
2.00
2.25 |
Source: AASHTO. Green Book. 2004 |
To calculate minimum superelevation runoff length, use the equation:
where:
L = minimum length of superelevation runoff (ft.)
G = maximum relative gradient (%)
N1 = number of lanes rotated (on one side of axis of rotation, not total number lanes)
bw = adjustment factor for number of lanes rotated
w = width of one traffic lane (usually 12-ft.)
ed = design superelevation rate (%)
For example, assume a five-lane roadway (12-ft. lanes) with 0.06 (6%) superelevation and 45 mph design speed. In the equation above, G = 0.54, N1 = 2.5, bw = 0.7, w = 12, and e = 6.0. Inserting these numbers into the equation gives:
