Sight Distance On Curve Calculator

Work out sight distance on curve instantly with clear inputs, formula shown and shareable results.

About Sight Distance On Curve Calculator

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.

Formulas

Required clearance

M = R (1 - cos(S / 2R)), angle in radians

Achievable sight distance

S = 2 R arccos(1 - available clearance / R)

Worked example

With the default values below, Sight Distance On Curve Calculator returns Clearance required inside the curve of 4.1570. Change any input above and every figure here is recalculated in your browser.

Inputs used

Curve radiusminimum 1 m: 300 m
Required sight distanceminimum 1 m: 100 m
Clearance available inside the curveminimum 0 m: 5 m

Results

Clearance required inside the curve: 4.1570 m
Sight distance achievable with the clearance available: 109.70 m
Clearance shortfall: 0.0000 m
Assessment: Sight distance is available

Step by step

1Values used
2Required clearance
3Achievable sight distance
4Clearance required inside the curve= 4.1570 m
5Sight distance achievable with the clearance available= 109.70 m
6Clearance shortfall= 0.0000 m
7Assessment= Sight distance is available

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Sight distance requirements and eye and object heights must be taken from the applicable geometric design standard.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Sight distance requirements and eye and object heights must be taken from the applicable geometric design standard.

Frequently Asked Questions

How is the Sight Distance On Curve figure worked out?

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.

What counts as an obstruction?

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.

What values do I need to enter?

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.