One 3.65 m lane rotated at 80 km/h
Using the displayed relative gradient 0.5%, e = 6% and normal crossfall 2%: Lr = 3.65 × 6/0.5 = 43.80 m and Lt = Lr × 2/6 = 14.60 m. No multi-lane adjustment is needed.
Calculate superelevation runoff, tangent runout and key stations for a selected road cross-section.
Effective width We = w · n1 · bw → Lr = We · ed / Δ (ed and Δ in %),
i.e. Lr = (w · n1 · ed · bw) / Δ.
Lt = (eNC / ed) · Lr.
For a ramp on a left-hand curve (crossfall already towards the inside): Lr = We · (ed − eNC) / Δ and Lt = 0.
Δ is the maximum relative gradient between the edge of the travelled way and the axis of rotation for the design
speed; bw reduces the length when more than one lane is rotated. Many agencies also avoid runoff shorter
than the distance travelled in about 2 s, for appearance.
highlighted = current design speed and superelevation.
Single carriageway (two-way). On the tangent the road has a normal crown (NC): both lanes fall at eNC away from the centreline. Over the tangent runout Lt the outside lane is raised until it is level (level crown, LC). The superelevation runoff Lr follows: at the reverse crown (RC) the whole section forms one plane at +eNC, and it keeps rotating about the centreline until the full superelevation (FS) ed is reached.
Interchange ramp (one-way). The whole ramp rotates as one unit about its right-hand edge, so the runoff is based on the full ramp width. Its normal crossfall is the same whichever way it turns: looking ahead, the left edge is low and the right edge high. On a left-hand curve this already falls to the inside (−eNC), so there is no tangent runout: the right-hand edge is the axis, the left-hand edge is lowered and Lr = We(ed − eNC) / Δ. On a right-hand curve it falls to the outside (−eNC), so it goes through the tangent runout to level and then the full runoff to ed: the right-hand edge is the axis and the left-hand edge is raised, Lr = We · ed / Δ.
Dual carriageway (divided). Each carriageway rotates about its median edge, so the width of the lanes in one direction is used. Both start with crossfall falling away from the median, like a crowned single carriageway. The outer carriageway goes through the tangent runout to level and on to ed; the inner carriageway keeps its crossfall until RC and is then steepened to ed.
Placing the runoff. The runoff is split between the tangent and the curve (60–80 % on the tangent without spirals) so drivers are not on a curve with too little superelevation, nor on a tangent with too much. Where spiral transitions are used, the runoff is usually placed over the length of the spiral. For a single or dual carriageway, a right-hand curve is the mirror image of the left-hand case.
Calculate superelevation runoff, tangent runout and key stations for a selected road cross-section.
Widths and lengths m; speed km/h; crossfall and relative gradient %.
Runoff Lr = We × e/Δ and tangent runout Lt = (normal crossfall/e)Lr. Percentage units cancel when e and Δ use the same convention. We includes the rotated width and the selected lane adjustment. The original tables cite AASHTO 2018 §3.3.8; confirm the governing table before design use.
Illustrative inputs for checking the method; these are not project records.
Using the displayed relative gradient 0.5%, e = 6% and normal crossfall 2%: Lr = 3.65 × 6/0.5 = 43.80 m and Lt = Lr × 2/6 = 14.60 m. No multi-lane adjustment is needed.
Using the displayed relative gradient 0.44%, e = 6% and normal crossfall 2%: Lr = 3.65 × 6/0.44 = 49.77 m and Lt = Lr × 2/6 = 16.59 m. No multi-lane adjustment is needed.
Calculate superelevation runoff, tangent runout and key stations for a selected road cross-section.
Widths and lengths m; speed km/h; crossfall and relative gradient %. Runoff Lr = We × e/Δ and tangent runout Lt = (normal crossfall/e)Lr. Percentage units cancel when e and Δ use the same convention. We includes the rotated width and the selected lane adjustment. The original tables cite AASHTO 2018 §3.3.8; confirm the governing table before design use.
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