AASHTO Stopping Sight Distance
Stopping sight distance per AASHTO Green Book (2018), 7th edition: SSD = brake reaction distance (2.5 s default) + braking distance (deceleration 11.2 ft/s²). Grade correction included for upgrades and downgrades.
Defaults: 55 mph, level grade. Compares computed SSD to AASHTO Green Book Table 3-1 design values (rounded for design-speed reasonableness).
What SSD is for
Stopping sight distance is the minimum distance over which a driver traveling at design speed must be able to see a 2-ft tall object on the road and stop before hitting it. SSD governs:
- Vertical curves: K-factor for crest curves is calibrated to provide SSD at design speed.
- Horizontal curves: lateral clearance (mid-ordinate distance) from the inside of the curve to obstructions must be ≥ M = R(1 − cos(28.65 SSD/R)).
- Intersection sight distance (different formula, longer than SSD).
- Passing sight distance (different formula, much longer than SSD).
AASHTO Green Book SSD design values
Pre-computed for level grade (G = 0):
- 25 mph: 155 ft
- 30 mph: 200 ft
- 35 mph: 250 ft
- 40 mph: 305 ft
- 45 mph: 360 ft
- 50 mph: 425 ft
- 55 mph: 495 ft
- 60 mph: 570 ft
- 65 mph: 645 ft
- 70 mph: 730 ft
- 75 mph: 820 ft
- 80 mph: 910 ft
Driver eye height and object height
SSD is measured from a driver eye height of 3.5 ft (1.08 m) — average for passenger cars — to an object 2 ft (0.60 m) tall. The 2-ft object represents tail-light height of a stopped vehicle. For trucks, eye height is 7.6 ft (2.33 m), which gives slightly longer SSD over crest curves but shorter SSD on horizontal curves with overhead obstructions.
Grade correction matters
On a 6% downhill, SSD increases ~25% over level for high-speed roadways. AASHTO requires using the steepest grade in the design segment, not an averaged value. For separated roadways with significant grade differential, compute SSD for each direction separately.
Ice, rain, fog
The AASHTO 11.2 ft/s² deceleration represents wet pavement performance for the 90th-percentile driver. For icy conditions, a much lower deceleration applies but is not the design basis (no infrastructure on Interstate roads is sized for ice). Local roadways in mountain corridors sometimes use 6 ft/s² for design near hazardous downhill segments.
Reference: AASHTO (2018). A Policy on Geometric Design of Highways and Streets (Green Book), 7th ed., §3.2.2 and Table 3-1. Garber, N.J., Hoel, L.A. (2015). Traffic and Highway Engineering, 5th ed., Cengage, ch. 15.