Stopping Sight Distance Calculator

Understanding the stopping sight distance calculator — concept and principle

Stopping sight distance (SSD) is the length of road a driver needs to see ahead in order to notice an obstacle and stop before reaching it. It is the most basic sight requirement in highway geometry: crest curves, curves near walls and intersections are all checked so that at least this distance is visible at the design speed.

Before the brakes even work, the car has already travelled 55.6 m.

How it is calculated

SSD = 0.278·V·t + V²/(254·(a/9.81 ± G))

Reaction distance: 0.278·V·t, the distance travelled during perception and reaction time t (s) at speed V (km/h); 0.278 converts km/h to m/s.
Braking distance: V²/(254·(a/9.81 ± G)), where a is the deceleration (m/s²) and G the grade as a decimal, positive uphill and negative downhill.

The AASHTO design values are t = 2.5 s and a = 3.4 m/s².

The principle

During the reaction time the car keeps its speed, so distance is simply speed times time. Braking from speed v to rest at constant deceleration covers v²/(2a). Writing v in km/h (v = V/3.6) gives V²/(25.92·a), and dividing a by g and multiplying by 2g·3.6² leads to the constant 254. On a grade, gravity helps braking uphill and hinders it downhill, which adds or subtracts G from a/g. AASHTO's 2.5 s exceeds the reaction time of about 90% of drivers in tests, and 3.4 m/s² is a deceleration most drivers can achieve on wet pavement while staying in their lane.

Worked example

Inputs: V = 80 km/h, t = 2.5 s, a = 3.4 m/s², grade 0%.
Reaction distance = 0.278 × 80 × 2.5 = 55.6 m.
a/9.81 = 0.34659, so braking distance = 80²/(254 × 0.34659) = 6400/88.03 = 72.70 m.
SSD = 55.6 + 72.70 = 128.30 m.

Each point is the calculator re-run with only “Design speed (km/h)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Design speed (km/h)	Stopping distance (m)
20	18.44377026
40	45.97508106
60	82.59393238
80	128.3003242
100	183.0942566
120	246.9757295
160	402.0012969

How to read the result

AASHTO rounds this calculated value up to a design SSD of 130 m for 80 km/h. Over half the distance comes from reaction time, which is why distraction is so dangerous. Braking distance grows with the square of speed: at 100 km/h it rises to about 114 m. A 3% downgrade increases the braking part from 72.7 m to about 79.6 m, while a 3% upgrade lowers it to about 66.9 m.

Common mistakes

Using a grade in percent inside the formula; the calculator divides by 100, but hand calculations must use 0.03, not 3.
Applying SSD where a longer decision sight distance is needed, such as complex interchanges.
Treating 3.4 m/s² as an emergency braking limit; it is a design assumption with margin.

Keep in mind: SSD values here follow the AASHTO model. Design must follow the road standard used in your country, which may use different reaction times, friction values and rounding, and must be checked by a qualified highway engineer.

Frequently asked questions

Why use 2.5 seconds of reaction time?

Studies found most drivers react faster; 2.5 s covers about 90% of drivers, including those surprised by an unexpected hazard.

Is SSD different for trucks?

Trucks brake less effectively, but their drivers sit higher and see farther, so the same SSD is generally applied.

How is the sight line measured?

In AASHTO, from a driver eye height of 1.08 m to an object 0.60 m high on the road.

Sources: AASHTO, A Policy on Geometric Design of Highways and Streets (Green Book) · NCHRP Report 400, Determination of Stopping Sight Distances, Transportation Research Board, 1997

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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