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Road design · AASHTO

Stopping Sight Distance Calculator

Stopping sight distance (SSD) is the length of road a driver needs to see ahead to stop for an object in the lane. Enter a design speed, brake reaction time, deceleration rate and grade to calculate SSD the way the AASHTO Green Book does — in US customary or metric units.

Units

AASHTO defaults: 2.5 s brake reaction time and 11.2 ft/s² (3.4 m/s²) deceleration.

Stopping sight distance

495 ftdesign value · calculated 492 ft

Brake reaction distance
202 ft
Braking distance
290 ft

Design values are rounded up to the next 5 ft (or 5 m), as in the AASHTO Green Book.

  • US customary & metric
  • Grade adjustment
  • Calculated & design values
  • AASHTO defaults built in
  • Full design tables

Definition

What Is Stopping Sight Distance?

Stopping sight distance is the sum of the brake reaction distance — how far a vehicle travels while the driver perceives an object and starts braking — and the braking distance needed to stop. A road must give drivers at least this much clear view ahead at every point, which is why it controls the design of crest curves, curves past buildings or cuttings, and sightlines at junctions.

AASHTO measures it from a driver’s eye height of 3.5 ft (1.08 m) to an object 2.0 ft (0.60 m) high on the road. The method is the same idea as the everyday stopping distance calculator, but with deliberately cautious inputs so the road works for slow-reacting drivers in poor conditions.

Formula

The AASHTO Stopping Sight Distance Formula

US customary

SSD = 1.47Vt + 1.075V²/a

SSD in feet · V = design speed (mph) · t = brake reaction time (2.5 s) · a = deceleration (11.2 ft/s²)

Metric

SSD = 0.278Vt + 0.039V²/a

SSD in metres · V = design speed (km/h) · t = brake reaction time (2.5 s) · a = deceleration (3.4 m/s²)

The first term is the brake reaction distance (1.47 converts mph to ft/s; 0.278 converts km/h to m/s). The second is the braking distance. The 2.5 s reaction time covers about 90% of drivers facing an unexpected object, and 11.2 ft/s² (about 0.35 g) is a deceleration roughly 90% of drivers exceed when braking hard, while staying in control on most wet surfaces.

On a grade, the braking term becomes V² ÷ (30 × (a ÷ 32.2 ± G)) in US units or V² ÷ (254 × (a ÷ 9.81 ± G)) in metric, with G the grade as a decimal — plus uphill, minus downhill. Design values are rounded up to the next 5 ft or 5 m.

Design values

Stopping Sight Distance Table

Level roads, 2.5 s brake reaction time and AASHTO deceleration — calculated with the formula above.

Stopping sight distance in US customary units
Design speedBrake reaction distanceBraking distanceCalculated SSDDesign SSD
15 mph55.1 ft21.6 ft76.7 ft80 ft
20 mph73.5 ft38.4 ft111.9 ft115 ft
25 mph91.9 ft60.0 ft151.9 ft155 ft
30 mph110.3 ft86.4 ft196.6 ft200 ft
35 mph128.6 ft117.6 ft246.2 ft250 ft
40 mph147.0 ft153.6 ft300.6 ft305 ft
45 mph165.4 ft194.4 ft359.7 ft360 ft
50 mph183.8 ft240.0 ft423.7 ft425 ft
55 mph202.1 ft290.3 ft492.5 ft495 ft
60 mph220.5 ft345.5 ft566.0 ft570 ft
65 mph238.9 ft405.5 ft644.4 ft645 ft
70 mph257.3 ft470.3 ft727.6 ft730 ft
75 mph275.6 ft539.9 ft815.5 ft820 ft
80 mph294.0 ft614.3 ft908.3 ft910 ft
Stopping sight distance in metric units
Design speedBrake reaction distanceBraking distanceCalculated SSDDesign SSD
20 km/h13.9 m4.6 m18.5 m20 m
30 km/h20.9 m10.3 m31.2 m35 m
40 km/h27.8 m18.4 m46.2 m50 m
50 km/h34.8 m28.7 m63.4 m65 m
60 km/h41.7 m41.3 m83.0 m85 m
70 km/h48.7 m56.2 m104.9 m105 m
80 km/h55.6 m73.4 m129.0 m130 m
90 km/h62.6 m92.9 m155.5 m160 m
100 km/h69.5 m114.7 m184.2 m185 m
110 km/h76.5 m138.8 m215.2 m220 m
120 km/h83.4 m165.2 m248.6 m250 m
130 km/h90.3 m193.9 m284.2 m285 m

Source formula: AASHTO, A Policy on Geometric Design of Highways and Streets (the “Green Book”), Chapter 3.

Grades

Stopping Sight Distance on Grades

Downhill, gravity adds to the braking distance; uphill, it helps you stop.

Design stopping sight distance on grades (US customary)
Design speed−9%−6%−3%+3%+6%+9%
30 mph230 ft215 ft205 ft190 ft185 ft180 ft
40 mph355 ft335 ft315 ft290 ft280 ft270 ft
50 mph510 ft475 ft450 ft405 ft390 ft375 ft
60 mph690 ft640 ft600 ft540 ft515 ft495 ft
70 mph895 ft825 ft775 ft690 ft660 ft635 ft

Calculated with AASHTO’s grade equation and rounded up to the next 5 ft. Many agencies use the level-road value on upgrades as a conservative choice.

FAQ

Stopping Sight Distance FAQ

What is stopping sight distance?

The length of road ahead a driver must be able to see in order to stop before reaching an object in the lane. It is the brake reaction distance plus the braking distance, and every point on a road should provide at least this much sight distance.

What is the AASHTO stopping sight distance formula?

In US customary units, SSD = 1.47Vt + 1.075V²/a, with V in mph, t the brake reaction time in seconds (2.5 s) and a the deceleration in ft/s² (11.2). In metric units, SSD = 0.278Vt + 0.039V²/a, with V in km/h and a in m/s² (3.4).

What is the stopping sight distance at 55 mph?

With AASHTO defaults on a level road, 202 ft of brake reaction distance plus 290 ft of braking gives 492 ft, which rounds up to a design stopping sight distance of 495 ft.

What perception–reaction time does AASHTO use for stopping sight distance?

2.5 seconds. It is long enough for about 90% of drivers to perceive an unexpected object and begin braking, which is far more conservative than the 0.7–1.5 s typical of alert drivers.

How does grade affect stopping sight distance?

Downgrades lengthen the braking distance because gravity works against the brakes; upgrades shorten it. AASHTO’s grade formula is d = V² ÷ (30 × (a ÷ 32.2 ± G)), with G the grade as a decimal — plus for uphill, minus for downhill.

What eye and object heights are used for stopping sight distance?

AASHTO measures stopping sight distance from a driver’s eye height of 3.5 ft (1.08 m) to an object 2.0 ft (0.60 m) high on the road — roughly the height of a vehicle’s tail lights.

What is the difference between stopping distance and stopping sight distance?

Stopping distance is how far a particular driver and vehicle need to stop in given conditions. Stopping sight distance is a road-design value: a cautious stopping distance, using a slow 2.5 s reaction and modest braking, that the road’s sightlines must provide.

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