Crest Vertical Curve Length for SSD (AASHTO)
AASHTO minimum crest vertical-curve length L for a stopping sight distance, both S<=L and S>L branches, with the K rate.
Example
You enter
- Algebraic grade difference A (%, |g2-g1|) 5
- Stopping sight distance S (ft) 570
- Sight constant C (2158 SSD, 2800 passing) 2158
You get
- Minimum curve length L 753
- Rate of vertical curvature K 151 ft/% (K = L/A)
Checked against published worked examples from: Michigan DOT.
Details, formula, and sources
Formula or tableL = A S^2 / C for S <= L; L = 2 S - C/A for S > L; C = 2158 (SSD crest) or 2800 (passing); K = L/A.
Source and editionThe AASHTO A Policy on Geometric Design of Highways and Streets (the Green Book) crest vertical-curve sight-distance minimums, by name.
Free accessThe AASHTO crest-curve length equations are published in the Green Book. AHJ, state DOT design manual, and the licensed civil engineer of record govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesA_pct, S_ft, C, L_ft, K_ft_per_pct
- Branch selection L = A S^2 / C when S <= L, else L = 2 S - C/A; the governing branch is chosen by the S vs L comparisonAASHTO Green Book
- Sight constant C = 2158 for the SSD crest (3.5 ft eye, 2.0 ft object); 2800 for passing sight distanceAASHTO Green Book
- Crest SSD only sag curves use headlight/comfort/drainage criteria, not this relationscope of this tile