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Calcimator

Sight Distance Calculator

Calculate stopping sight distance (SSD), decision sight distance, and passing sight distance for highway design using AASHTO formulas.

Inputs

mph

AASHTO Green Book Table 3-1: 30 mph SSD=200 ft; 45 mph=360 ft; 60 mph=570 ft; 70 mph=730 ft

sec

AASHTO Table 3-1: 20 mph f=0.40; 45 mph f=0.35; 65 mph f=0.30; 70 mph f=0.28

%

Results

Stopping Sight Distance

358 ft

≈ 9 school buses

Passing Sight Distance

383 ft

≈ 10 school buses

Decision Sight Distance656 ft
Brake Reaction Distance165 ft
Braking Distance193 ft
Meets AASHTO Standard1
How to Use This Calculator
  1. Enter design speed (mph), perception-reaction time (seconds), and friction coefficient.
  2. Set roadway grade (%).
  3. Review Stopping Sight Distance (ft), Decision Sight Distance (ft), and Passing Sight Distance (ft).
  4. Use these values to verify horizontal and vertical alignment clearances meet AASHTO standards.

How the result changes with Design Speed

Design SpeedStopping Sight DistancePassing Sight Distance
22127 ft187 ft
38277 ft323 ft
57519 ft485 ft
74793 ft629 ft

What each input means

Design Speed
Design speed of the roadway per AASHTO Green Book. Design speed determines required SSD: 30 mph→200 ft; 45 mph→360 ft; 60 mph→570 ft; 70 mph→730 ft.
Perception-Reaction Time
Time for driver to perceive a hazard and begin braking. AASHTO standard is 2.5 seconds.
Friction Coefficient (f)
Pavement friction coefficient for wet conditions per AASHTO Green Book Table 3-1. Values decrease with speed: 20 mph→0.40; 45 mph→0.35; 65 mph→0.30; 70 mph→0.28.
Grade
Roadway grade in percent. Positive for uphill (shorter SSD), negative for downhill (longer SSD).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Design Speed = 45, Perception-Reaction Time = 2.5, Friction Coefficient (f) = 0.35, Grade = 0 = 4 input(s) provided
  2. Calculate Stopping Sight Distance
    Stopping Sight Distance
    358 = 358
  3. Calculate Passing Sight Distance
    Passing Sight Distance
    383 = 383
  4. Calculate Decision Sight Distance
    656 = 656
  5. Calculate Brake Reaction Distance
    Brake Reaction Distance
    165 = 165

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