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Stopping Sight Distance Calculator

Calculate stopping and decision sight distances, including the effect of ascending and descending road grades.

by Shahid Pervaiz: pervaiz.shahid@gmail.com
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Reference: AASHTO Green Book, 7th Ed. (2018) §3.2 Tables: 3-1 (SSD) · 3-2 (SSD on grades) · 3-3 (DSD)
Stopping Sight Distance — AASHTO · Tables 3-1 & 3-2
Design Speed
Design Speed V (kph)
Driver & Vehicle
Brake reaction time t (s)
Deceleration rate a (m/s²)
AASHTO values: t = 2.5 s (90th percentile driver), a = 3.4 m/s² (comfortable for most drivers, wet pavement).
Grade — Ascending & Descending (Table 3-2)
Grade G (%)
Choose the grade in the direction of travel: − descending (downgrade, gravity opposes braking → longer) or + ascending (upgrade, gravity helps braking → shorter).
A two-way road is driven both ways on the same slope, so the opposite direction is shown too and the descending value governs. On a divided road use the grade of each carriageway.
Calculation
Brake reaction distance (m) 69.5
Braking distance descending −3 % (m) —
Braking distance ascending +3 % (m) —
Braking distance level, 0 % (m) — reference 114.7
SSD calculated: level / ascending / descending (m) 184.2
Design Stopping Sight Distance (m) 185 m
Opposite direction (m) —
Sketch — Stopping Sight Distance on the Selected Grade (not to scale)
Computed Values
Design Speed100 kph
Reaction Distance69.5 m
Braking Descending—
SSD Level185 m
SSD Ascending—
SSD Descending—
Grade Effect vs Level—

SSD = 0.278·V·t + 0.039·V²/a  (AASHTO Eq. 3-2), V in kph, t in s, a in m/s².
On a grade: SSD = 0.278·V·t + V² / [254·((a/9.81) ± G)]  (Eq. 3-3), G as a decimal (+ ascending / upgrade, − descending / downgrade). Design values are the calculated distance rounded up to the next 5 m.
The grade changes only the braking part; the brake reaction distance is the same in both directions.
Verified: V = 100 kph, t = 2.5 s, a = 3.4 m/s² → 69.5 + 114.7 = 184.2 → SSD = 185 m ✓ (Table 3-1)

Stopping Sight Distance on Level Roadways — AASHTO Table 3-1

Calculated with the t and a entered on the left (AASHTO defaults reproduce Table 3-1). highlighted = current selection.

Stopping Sight Distance on Grades — AASHTO Table 3-2 (m)
When to Use Stopping vs Decision Sight Distance

Stopping sight distance is the minimum that must be available at every point on every road, measured from a driver's eye height of 1.08 m to an object 0.60 m high. It governs crest and sag vertical curves and horizontal sight lines.
Decision sight distance should be provided where drivers must make complex or unexpected decisions: interchange and intersection approaches, lane drops, changes in cross-section, toll plazas, and areas with visual clutter or concentrated demand for information. Where DSD cannot be provided, advance warning signs and markings should be considered.

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Method & practical use

Understand the result.

Calculate stopping and decision sight distances, including the effect of ascending and descending road grades.

How to use this tool

  1. Select the geometry or design case and confirm the displayed units.
  2. Replace sample inputs with the project values, then calculate or review the live result.
  3. Check the method, assumptions and output; save or print the calculation with its input record.

Units & method

Speed km/h, distances m, reaction time s, deceleration m/s², grade %.

SSD combines reaction distance 0.278Vt with braking distance. On grade, braking distance is V²/[254(a/9.81 + G)], with G signed and expressed as a decimal. Default t = 2.5 s and a = 3.4 m/s²; displayed design values round up to 5 m. Check the governing edition for decision-manoeuvre tables.

Worked examples

Illustrative inputs for checking the method; these are not project records.

60 km/h on level ground

Using t = 2.5 s and a = 3.4 m/s² gives 82.99 m, rounded upward to 85 m for the displayed design distance.

100 km/h on level ground

Using t = 2.5 s and a = 3.4 m/s² gives 184.21 m, rounded upward to 185 m for the displayed design distance.

Frequently asked questions

What does this tool calculate or track?

Calculate stopping and decision sight distances, including the effect of ascending and descending road grades.

Which units and assumptions should I use?

Speed km/h, distances m, reaction time s, deceleration m/s², grade %. SSD combines reaction distance 0.278Vt with braking distance. On grade, braking distance is V²/[254(a/9.81 + G)], with G signed and expressed as a decimal. Default t = 2.5 s and a = 3.4 m/s²; displayed design values round up to 5 m. Check the governing edition for decision-manoeuvre tables.

Can I save or share the result?

Use the save, export and print controls shown in this workspace. Where CSV export is available, the exported data can be opened in a spreadsheet. Browser print can save a PDF.

Continue your workflow

How to use the Stopping & Decision Sight Distance Calculator

Everything runs in your browser — nothing is uploaded. If you are new to the tool, follow these steps in order.

1

Enter your inputs and read the results

Choose a calculation with the two buttons at the top: ⏹ Stopping Sight Distance or ↷ Decision Sight Distance. Results update as you type, so there is no Compute button to press.

  • Project / road name and Prepared by — shown on the printout and report, and used to name saved files.
  • Stopping Sight Distance — enter design speed, reaction time, deceleration and grade. It gives the reaction and braking distances and the stopping sight distance on level, ascending and descending grades.
  • Decision Sight Distance — choose the avoidance manoeuvre (A to E). It gives the decision sight distance on level, ascending and descending grades.
  • Sketches and tables — the selected case on the grade, and the AASHTO tables for comparison.
2

Save your project and open it later

💾 Save Project (.json) saves every input on the page in one .json project file. 📁 Load Project opens that file again, on any computer.

The note beside the Save button tells you whether you have unsaved changes or when you last saved. If you try to close or leave the page with unsaved changes, the browser asks you to confirm first.

Use the project file for backups and for sharing with colleagues. Don't edit it by hand.

This tool does not autosave. Save your project before closing the page.

3

Print, PDF and report

  • 🖨 Print / Save as PDF prints the calculation on screen: inputs, results and sketch. To get a PDF, choose Save as PDF as the printer in the print dialog.
  • 📄 Generate Report opens the full design report for both calculations in a new tab, with its own Print / Save as PDF button. If nothing opens, allow pop-ups for this site.
4

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