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Braking Distance & Stopping Sight Distance (AASHTO) Calculator

Stopping Sight Distance (SSD) is the minimum distance required on a roadway for a motorist traveling at design speed to perceive, react, and bring the vehicle to a controlled stop before hitting a stationary object.

Operating speed of vehicle in kilometers per hour.

Driver perception and reaction time (AASHTO design standard = 2.5s; alert driver = 1.5s).

Tire-to-roadway friction coefficient factor.

Longitudinal road slope (+% uphill reduces stopping distance, -% downhill increases distance).

Calculated Result
111.76 m

Total Stopping Distance

Total Stopping Sight Distance

111.76 m (366.7 ft)

Driver Reaction Distance

55.56 m (@ 2s)

Vehicle Braking Distance

56.2 m

Vehicle Speed

100 km/h (62.1 mph / 27.78 m/s)

Tire-Road Friction (f)

0.7 (dry-asphalt)

Calculation Breakdown

  1. 1. Velocity Conversion & Reaction Distancev = 100 km/h = 27.78 m/s. Perception-reaction distance d_r = v · t_r = 27.78 m/s · 2s = 55.56 m
  2. 2. Braking Distance Calculationd_b = v² / [2 · g · (f + G)] = (27.78)² / [2 · 9.81 · (0.7 + 0)] = 56.2 m
  3. 3. Total Stopping DistanceSSD = d_r + d_b = 55.56 m + 56.2 m = 111.76 meters (366.7 feet)

Stopping Distance by Road Condition (100 km/h)

Interactive visualization based on your current inputs

Distance (m)
0.0112224337449Dry AsphaltWet AsphaltPacked SnowGlare IceSurface ConditionDistance (m)

What Is the Braking Distance & Stopping Sight Distance (AASHTO) Calculator?

Stopping Sight Distance is the sum of distance traversed while a driver spots a hazard and activates brakes plus physical vehicle braking distance.

How Does the Braking Distance & Stopping Sight Distance (AASHTO) Calculator Work?

Equates vehicle initial kinetic energy to work performed by tire-road friction forces over stopping distance.

Braking Distance & Stopping Sight Distance (AASHTO) Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

d_r = v \cdot t_r, \quad d_b = \frac{v^2}{2 g (f \pm G)}, \quad \text{SSD} = d_r + d_b

Total stopping distance sums reaction distance (speed times reaction time) and braking distance (speed squared over 2g times friction plus grade).

How to Use the Braking Distance & Stopping Sight Distance (AASHTO) Calculator

  1. Enter travel speed in km/h.
  2. Specify perception-reaction time (seconds).
  3. Select roadway pavement surface condition.
  4. Optionally specify vertical hill grade percentage.

Step-by-Step Example Calculation

100 km/h Highway Stop on Wet Asphalt (2.0s Reaction)

Input Values:

speedKmh:100
reactionTimeSeconds:2.0
roadCondition:wet-asphalt
roadGradePercent:0
Worked Steps: Driver reaction distance is 55.56 m. Braking distance is 112.41 m, giving a total Stopping Sight Distance of 167.97 m (551.1 ft).

Understanding Your Result

Total Stopping Distance: Combined distance needed to come to a complete stop.

Reaction Distance: Distance traveled before brakes begin physically decelerating vehicle.

Braking Distance: Skid distance under ABS or tire friction.

Factors That Affect the Result

  • Speed (quadratic impact), road surface contamination (oil/wet/ice), tire tread wear, and downhill grades.

When Should You Use This Calculator?

  • Highway horizontal and vertical alignment design, accident reconstruction analysis, and traffic safety audits.

Assumptions & Limitations

  • Assumes functioning braking system capable of maintaining vehicle at peak friction limits without mechanical brake fade.

Frequently Asked Questions

Calculation Accuracy & Reference Note

Standard AASHTO Green Book geometric highway stopping sight distance model.

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