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
Details, formula, and sources
AASHTO middle-ordinate clear-zone M = R(1 - cos(28.65 S/R)) an inside obstruction must clear for stopping sight distance.
M = 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.
The AASHTO A Policy on Geometric Design of Highways and Streets (the Green Book) horizontal sightline offset (middle ordinate), by name.
The AASHTO middle-ordinate relation is published in the Green Book. AHJ, state DOT design manual, and the licensed civil engineer of record govern.
Estimate. AHJ and licensed professional govern.
Field names used by the API: mode, 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