Stopping Sight Distance (AASHTO)
AASHTO Green Book SSD = perception-reaction + braking distance from speed, friction, and grade.
Example
You enter
- Speed (mph) 55
- Perception-reaction time (s; default 2.5) 2.5
- Friction coefficient f (set from condition or enter directly) 0.348
You get
- Perception-reaction distance (ft) 202.1
- Braking distance (ft) 290.3
- Total SSD (ft) 492.4
Checked against published worked examples from: Illinois DOT (AASHTO Green Book values).
Details, formula, and sources
ScopeState DOT design tables round these numbers.
Formula or tabled_pr = 1.47 * v * t_pr; d_br = 1.075 v^2 / (32.2 (f + g)), the Green Book form (a = 11.2 ft/s^2 at f = 0.348; 290.3 ft at 55 mph); d = d_pr + d_br.
Source and editionAASHTO Green Book (Policy on Geometric Design of Highways and Streets, 7th ed.) Chapter 3.
Free accesstransportation.org for TOC; AASHTO design SSD tables are licensed.
What governsMath aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
API field namesspeed_mph, reaction_time_s, friction, perception_reaction_ft, braking_distance_ft, total_ssd_ft
- Perception-reaction time 2.5 s defaultAASHTO Green Book Chapter 3
- Friction coefficient 0.348 AASHTO design (11.2 ft/s^2 / 32.2, already a wet-pavement basis) / 0.20 poor wet surface or worn tires (not an AASHTO value) / 0.10 ice or packed snowAASHTO Green Book Chapter 3 for 0.348; engineering practice for 0.20 and 0.10
- Grade decimal; + uphill, - downhillstandard convention