How to use the Stopping Sight Distance Calculator
- 1. Enter design speedSpeed in km/h. Reaction time and friction default to common IRC values.
- 2. Add the gradientPositive for an ascending grade, negative for descending.
- 3. Get SSDLag distance, braking distance, SSD, ISD and the grade cases.
SSD formula
SSD = 0.278·V·t + V²/(254(f ± G)), with V in km/h, t in seconds, f the longitudinal friction coefficient and G the gradient as a decimal (+ up, − down). In m/s the same formula is v·t + v²/(2g(f ± G)).
Commonly used IRC values: reaction time 2.5 s, friction from 0.40 at low speeds down to 0.35 at 80 km/h and above. On a single-lane road with two-way traffic the sight distance is 2 × SSD. Intermediate sight distance (ISD) is also 2 × SSD. Check the current IRC code for design tables.
Worked example and exam tips
V = 80 km/h, t = 2.5 s, f = 0.35, level road: lag = 0.278 × 80 × 2.5 = 55.6 m, braking = 80²/(254 × 0.35) = 72.0 m, SSD = 127.6 m. On a 3% downgrade the braking distance becomes 80²/(254 × 0.32) = 78.7 m.
- For two-way traffic on a grade, design for the descending direction (larger SSD).
- Write the units of V carefully: 0.278 converts km/h to m/s.
- IRC tables round SSD up, so they can be slightly higher than the formula.
Frequently asked questions
What is stopping sight distance?
The minimum distance a driver must see ahead to stop safely before hitting an object. It is the lag distance during reaction time plus the braking distance.
Why is reaction time taken as 2.5 s?
It covers perception, intellection, emotion and volition (PIEV) time for most drivers. 2.5 s is the value commonly used in IRC; check the current code.
How does gradient change SSD?
On an upgrade gravity helps braking, so SSD is shorter. On a downgrade it is longer. The tool shows both.
Is this calculator free?
Yes. You fill a short form once to see your result, and other Uncodemy tools then open without the form. Everything is calculated in your browser and nothing you type is uploaded.