Clearance needed on the inside of horizontal curves so that walls, cut slopes, buildings, vegetation or barriers do not block the stopping sight distance, and a check of the sight distance available past an existing obstruction, per the AASHTO Green Book.
HSO = R·[1 − cos(28.65·S / R)] (sight distance shorter than the curve), angle in degrees,
R = radius to the centreline of the inside lane (m), S = stopping sight distance (m).
When the curve is shorter than S: HSO = R·[1 − cos(28.65·L / R)] + ½(S − L)·sin(28.65·L / R).
The sight line runs from a 1.08 m eye height to a 0.60 m object, so at the obstruction it is about
0.84 m above the road: anything higher than that within the HSO blocks the view.
Stopping sight distance for each speed on level grade with the t and a entered on the left. highlighted = current speed column. "—" = sight line longer than half the circle (use a larger radius).
The offset is measured from the centreline of the inside lane (the driver's path) to the obstruction, at the middle of the sight line. On a two-lane road with the radius given to the road centreline, the radius to the inside lane is Rc − w/2. The clear distance from the inside edge of the travelled way is HSO − w/2 — this is the width that must be kept clear of walls, cut slopes, buildings, parked vehicles, crops and vegetation higher than about 0.84 m. Where the offset cannot be provided, consider flattening the curve, widening the cut, relocating the obstruction or, as a last resort, a lower design speed with appropriate signing.
It is the clear distance needed on the inside of a horizontal curve, measured from the centreline of the inside lane, so that nothing blocks the driver's view over the stopping sight distance. It is also called the middle ordinate or lateral clearance.
AASHTO gives HSO = R·[1 − cos(28.65·S/R)], where R is the radius to the centreline of the inside lane and S the stopping sight distance, with the angle in degrees. This applies when the sight distance is shorter than the curve. When the curve is shorter, part of the sight line lies on the tangents and the general formula shown on this page is used.
Use Check Available Sight Distance: enter the curve radius and the clearance to the obstruction. The tool gives the sight distance available, S = (R/28.65)·cos⁻¹[(R − M)/R], compares it with the stopping sight distance required, and shows the highest design speed that clearance supports.
Yes. The stopping sight distance is longer on a descending grade, so the required offset is larger; on an ascending grade it is smaller. On two-way roads the descending direction usually governs, and the tool shows both.
The method follows A Policy on Geometric Design of Highways and Streets, 7th Edition (2018), sight distance on horizontal curves, with the stopping sight distance criteria of Section 3.2. Always confirm against the edition adopted by your road authority.