The following excerpt was taken from the 1994 edition of AASHTO's A
Policy on Geometric Design of Highways and Streets (p. 219).
Another element of horizontal alignment is the sight distance across the inside of
curves. Where there are sight obstructions (such as walls, cut slopes, buildings, and
longitudinal barriers) on the inside of curves, a design to provide adequate sight
distance may require adjustment in the normal highway cross section or change in alignment
if the obstruction cannot be removed.
Because of the many variables in alignment and cross sections and in number,
type, and location of possible obstructions, specific study usually is necessary
for each condition. Using design speed and a selected sight distance as a control,
the designer should check the actual condition and make the necessary adjustments
in the manner most fitting to provide adequate sight distance.
Stopping Sight Distance
For general use in design of a horizontal curve, the sight line is a chord of the
curve, and the applicable stopping sight distance is measured along the centerline of the
inside lane around the curve.