# Road design

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### 7.7.6 Length of Superelevation Development

The length required to develop superelevation should be adequate to ensure a good appearance and give satisfactory riding qualities. The higher the speed or wider the carriageway, the longer the superelevation development will need to be to meet the requirements of appearance and comfort.

The length of superelevation development is the transition of crossfall from a normal roadway on straight alignment to that of a fully superelevated crossfall on a circular curve (refer to Figure 7.10). The total length required to develop superelevation is called the overall length of superelevation development (Le). It consists of two main elements:

• superelevation runoff (Sro) the length of roadway needed to accomplish a change in crossfall from flat crossfall to a fully superelevated crossfall
• tangent runout (Tro) is the length of roadway required to accomplish the change in crossfall from a normal crown section to a flat crossfall.

Lengths of superelevation development are determined from the two design criteria of:

• rate of rotation of the pavement crossfall
• relative grade of the axis of rotation to the edges of carriageway grades being rotated.

Superelevation runoff and tangent runout lengths are calculated by proportioning the normal crossfall to full superelevation using design values for superelevation development shown in Table 7.11.

 Sro = $\mathrm{L}_{\mathrm{e}}\mathrm{- }\mathrm{L}_{\mathrm{e}}\left\lbrack \frac{\mathrm{e}_{\mathrm{1}}}{\mathrm{e}_{\mathrm{1}}\mathrm{+ }\mathrm{e}_{\mathrm{2}}} \right\rbrack$ 11 Tro = $\mathrm{L}_{\mathrm{e}}\mathrm{- }\mathrm{S}_{\mathrm{\text{ro}}}$ 12 where Le = superelevation development length (m) Sro = superelevation runoff (m) Tro = tangent runout (m) e1 = normal crossfall (%) e2 = full superelevation crossfall (%)

For appearance purposes some road agencies (e.g. Main Roads Western Australia) may choose to use a rounding vertical curve to ease the grade changes from crossfall to superelevation at the edges of the pavement and formation. In this case the superelevation transition length is the superelevation development length including the rounding vertical curves, and the rounding vertical curve is denoted as the ‘ease’. This approach is shown in Figure 7.14 to Figure 7.21.

Rounding curve lengths are shown in Table 7.9. Note that these rounding curve lengths are not applicable in situations where stopping sight distance to a zero object height is required (i.e. at intersections) and in this situation they should be amended accordingly.

Table 7.9: Superelevation development length rounding curve length

Number of lanes in one direction
Length of rounding (m)
1 (3.5 m)20
2 (7.0 m)30
3 (10.5 m)40

Source: Main Roads Western Australia (2015a).

Figure 7.10: Typical superelevation development profile on two lane roads (tangent to transition curve to circular curve)