Work out sight distance on curve instantly with clear inputs, formula shown and shareable results.
On a horizontal curve the sight line is a chord, so an obstruction inside the curve blocks visibility. The clearance needed is the middle ordinate of that chord, R(1 - cos(S / 2R)) with the angle in radians. Working backwards from the clearance actually available gives the sight distance that can be achieved.
Required clearance
M = R (1 - cos(S / 2R)), angle in radians
Achievable sight distance
S = 2 R arccos(1 - available clearance / R)
Preliminary estimate only. Sight distance requirements and eye and object heights must be taken from the applicable geometric design standard.
M = R (1 - cos(S / 2R)), angle in radians. On a horizontal curve the sight line is a chord, so an obstruction inside the curve blocks visibility.
Anything at driver eye height to object height along the sight line: cut slopes, walls, parapets, planting and parked vehicles. The clear zone must be maintained, not just built once.
This calculator takes 3 inputs: Curve radius, Required sight distance, Clearance available inside the curve. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.