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) 0.35
You get
- Perception-reaction distance (ft) 202.1
- Braking distance (ft) 288.1
- Total SSD (ft) 490.2
Details, formula, and sources
State DOT design tables round these numbers.
d_pr = 1.47 * v * t_pr; d_br = v^2 / (30 * (f + g)); d = d_pr + d_br.
AASHTO Green Book (Policy on Geometric Design of Highways and Streets, 7th ed.) Chapter 3.
transportation.org for TOC; AASHTO design SSD tables are licensed.
Math aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
Field names used by the API: speed_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 0.35 dry / 0.20 wet / 0.10 iceengineering practice; AASHTO design values
- Grade decimal; + uphill, - downhillstandard convention