Pavements & traffic · Road geometry
Stopping sight distance and vertical curves calculator
Stopping sight distance for the design speed, reaction time and deceleration, corrected for the grade; the crest curve length that provides that sight distance between driver eye and object heights (both S < L and S > L cases), the sag curve length for headlight sight distance, and the K values. The profile is drawn with the sight line.
Standard: Austroads Guide to Road Design Part 3 (SSD = RT·V/3.6 + V²/254(d + 0.01a)) and Part 3 Sec 8 vertical curves (crest sight distance, sag headlight distance)
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Design speed | 100 | km/h |
| Reaction time | 2 | s |
| Deceleration coefficient d | 0.36 | |
| Approach grade g1 | 3 | % |
| Departure grade g2 | -2.5 | % |
| Driver eye height | 1.1 | m |
| Object height (0.2 car / 0.8 tail-light) | 0.2 | m |
| Headlight height (sag) | 0.65 | m |
| Headlight beam upward angle | 1 | ° |
| Proposed curve length | 380 | m |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Grade difference A | -5.50 | % |
| Stopping sight distance SSD | 173.1 | m |
| Crest curve length for SSD | 368 | m |
| K value (L/A) required | 66.9 | m/% |
| Proposed length / K | 380 m / 69.1 | |
| Sight distance on the proposed crest | 176 m |
- ✓ Proposed length ≥ required — 380 vs 368 m
- ✓ Minimum length ≥ 0.6 × V for appearance
Method, formulas and assumptions
- SSD = RT·V/3.6 + V²/[254(d + 0.01a)] with a the (negative) downgrade in %.
- Crest: S < L: L = A·S²/[200(√h1 + √h2)²]; S > L: L = 2S − 200(√h1 + √h2)²/A.
- Sag (headlight): S < L: L = A·S²/[200(h + S·tanθ)]; S > L: L = 2S − 200(h + S·tanθ)/A.
- Check also comfort (sag K ≥ V²/1296a) and drainage at the low point.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.