Vehicle Braking Distance
Stopping distance from speed, friction, grade, reaction time.
Example
You enter
- Speed (mph) 55
- Road friction coefficient 0.7
- Reaction time (s) 1.5
You get
- Braking distance 144.1 ft
- Reaction distance 121.0 ft
- Total stopping distance 265.1 ft
- Effective friction 0.7
Checked against published worked examples from: Gary A. Presswood, ScD, PE...
Details, formula, and sources
Formula or tableBraking distance d = v² / (2 × g × (μ + grade)) + reaction_distance. Reaction distance = v × t_reaction. g = 32.2 ft/s².
Source and editionClassical kinematics; AASHTO 'Policy on Geometric Design of Highways and Streets' (Green Book) by name.
Free accessAASHTO Green Book licensed; kinematics free in physics texts.
What governsEstimate. AHJ and licensed fire protection engineer govern.
API field namesspeed_mph, friction_coefficient, reaction_time_s, braking_distance_ft, reaction_distance_ft, total_distance_ft, effective_friction
- Typical friction μ (enter one) 0.7 dry asphalt / 0.4 wet / 0.2 ice; the AASHTO Green Book's stopping sight distance uses a fixed 11.2 ft/s² deceleration (about μ = 0.35) insteadcommon accident-reconstruction values; AASHTO Green Book for the design deceleration
- Default reaction time 1.5 s unless user supplies, an alert driver expecting to stop; the AASHTO Green Book design perception-reaction time is 2.5 s, which adds 1.0 s x 1.467 ft/s per mph (about 81 ft at 55 mph)AASHTO Green Book (2.5 s design value)