Horizontal Sightline Offset on a Curve (AASHTO)
How far back an inside obstruction must sit on a curve for stopping sight distance.
Example
You enter
- Solve for M
- Curve radius R to inside-lane CL (ft) 1000
- Sight distance S (ft) 570
You get
- M (ft) 40.3
Checked against published worked examples from: Michigan DOT.
Details, formula, and sources
ScopeAASHTO middle-ordinate clear-zone M = R(1 - cos(28.65 S/R)) an inside obstruction must clear for stopping sight distance.
Formula or tableM = R (1 - cos(28.65 S / R)) [half-angle in degrees]; inverse S = (R/28.65) arccos(1 - M/R); R to the inside-lane centerline.
Source and editionThe AASHTO A Policy on Geometric Design of Highways and Streets (the Green Book) horizontal sightline offset (middle ordinate), by name.
Free accessThe AASHTO middle-ordinate relation is 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 namesmode, R_ft, S_ft, M_ft
- Middle-ordinate model M = R (1 - cos(28.65 S / R)), the 28.65 = 90/pi putting the half-angle in degreesAASHTO Green Book
- Radius basis R is to the inside-lane centerline (the vehicle's path), not the curve centerlineAASHTO Green Book
- Continuous obstruction assumes the sight obstruction runs continuously along the curve; no end geometryscope of this tile