Stopping Sight Distance and Horizontal Curve Design As is the case for vertical curve design, adequate SSD must be provided in the design of horizontal curves. Sight distance restrictions on horizontal curves occur when obstructions are present. Such obstructions are frequently encountered in highway design due to the cost of right-of-way acquisition or the cost of moving earthen materials, such as rock outcroppings. When such an obstruction exists, the SSD is measured along the horizontal curve from the center of the traveled lane (the assumed location of the driver’s eyes). for a specified stopping distance, some distance Ms (the middle ordinate of a curve that has an arc length equal to the SSD) must be visually cleared so that the line of sight is such that sufficient SSD is available.
Stopping sight distance considerations for horizontal curves.
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R = radius measured to the centerline of the road in ft,
Ms = middle ordinate necessary to provide adequate stopping sight distance (SSD) in ft.
Rv = radius to the vehicle’s traveled path (usually measured to the center of the innermost lane of the road) in ft,
SSD = stopping sight distance in ft,
PC = point of curve (the beginning point of the
Δ = central angle of the curve in degrees, horizontal curve), and
Δs = angle (in degrees) subtended by an arc equal in length to the required stopping sight distance (SSD),
PT = point of tangent (the ending point of the horizontal curve).
L = length of curve in ft,
Equations for computing SSD relationships for horizontal curves can be derived by first determining the angle, ∆s, for an arc length equal to the required SSD. Assuming that the length of the horizontal curve exceeds the required SSD