Underpass Sight Distance and Sag Vertical Curve Design

Underpass Sight Distance and Sag Vertical Curve Design. design for sag curves is based on nighttime conditions because during daytime conditions a driver can see the entire sag curve. However, in the case of a sag curve being built under an overhead structure (such as roadway or railroad crossing), a driver’s line of sight may be restricted so that the entire curve length is not visible.

In designing the sag curve, it is essential that the curve be long enough to provide a suitably gradual rate of curvature such that the overhead structure does not block the line of sight and allows the required SSD for the specified design speed to be maintained.

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Used by permission from American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, 7th Edition, Washington, DC, 2018.

S = sight distance in ft, G1 = initial roadway grade in percent or ft/ft,

H1 = height of driver’s eye in ft, G2 = final roadway grade in percent or ft/ft,

H2 = height of object in ft, PVC = point of the vertical curve (the initial point of the curve), and Hc = clearance height of overpass structure

above roadway in ft, PVT = point of vertical tangent, which is the final point of the vertical curve (the point where the curve returns to the final grade or, equivalently, the final tangent).

L = length of the curve in ft

Again, by using the properties of a parabola for an equal-tangent vertical curve, it can be shown that the minimum length of sag curve for a required sight distance and clearance height