I’m seeking clarification on the recommended approach when coding horizontal curves with adverse crossfall, particularly where the curve radius is relatively large and, at first glance, may not appear critical in terms of crash risk.
As per international road design standards, it is permissible to apply adverse crossfall on flatter curves, provided the radius is large enough. However, from a safety risk perspective, the combination of curvature and adverse crossfall can introduce significant hazards—particularly in wet conditions—due to reduced lateral support and lower available side friction.
When viewing the curvature alone, the default inclination might be to code the curve as ‘Straight or gently curving’ based on its apparent radius. However, this would not adequately capture the increased crash risk presented by the adverse crossfall condition.
To address this, my approach has been as follows:
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Calculate the design speed of the curve using:
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The existing adverse crossfall, and
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A reduced side friction factor to reflect real-world surface conditions.
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Convert that calculated design speed back into an equivalent horizontal radius using standard assumptions for positive crossfall and standard side friction values.
This method appears to yield a more representative radius and results in a curve risk rating that aligns better with observed safety risks in the field.
My question to the forum is:
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Does this approach align with iRAP’s established methodology for coding horizontal alignment, particularly in situations involving adverse crossfall?
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If not, is there a recommended process within iRAP’s framework to appropriately account for the risk implications of adverse crossfall on curves?
I appreciate any guidance or references to relevant sections within the iRAP coding manual or methodology documents.