WORLD FOR CIVIL

Highway Alignments

The Geometric design of a highway consist of horizontal alignment, vertical alignment and cross-sectional elements.

Horizontal alignment of a highway defines its location and orientation in plan view. Vertical alignment of a highway deals with its shape in profile.

Stationing

Distance along a horizontal alignment is measured in terms of stations. A full station is defined as 100 ft (30.48 m) and a half station as 50 ft (15.24 m). This distance is measured horizontally along the centerline of the roadway, whether it is a tangent, a curve, or a combination of these.

Stopping Sight Distance

Stopping Sight Distance or SSD is the distance needed
between a vehicle and an arbitrary object (at some point down the road) to permit a driver to stop a vehicle safely before reaching the obstruction.
For crest vertical curves, AASHTO defines the minimum length L_min
, ft (m), of crest vertical curves based on a required sight distance S, ft (m), as that given by

1) For S
L_min=(AS²)/[100 (?2H1 +?2H2)²]

2)For S>L
L_min=25-[200((?H1+?H2)²]/AS²

where
A= algebraic difference in grades, percent, of the tangents to the vertical curve

H1= eye height, ft (m), above the pavement

H2= object height, ft (m), above the pavement

Design controls for vertical curves can be established in terms of the rate of vertical curvature K defined by

K=L/A

where
L= length, ft (m), of vertical curve
. K is useful in determining the minimum sight distance, the length of a vertical curve from the PVC to the turning point (maximum point on a crest and minimum on a sag). This distance is found by multiplying K by the approach gradient.
Recommended values of K for various design velocities and stopping sight distances for crest and sag vertical curves are published by AASHTO.