The Geometric design of a highway combine of horizontal alignment, vertical alignment and cross-sectional elements.
Horizontal alignment of a highway allocate its location and orientation in plan view. Vertical alignment of a highway handling with its shape in profile.
Stationing
Distance along a horizontal alignment is measured in condition of stations. A full station is defined as 100 feet (30.48 m) and a half station as 50 feet (15.24 m). This distance is measured horizontally alongside of the centerline of the roadway, whether it is a tangent, a curve, or a combination of these.
Stopping Sight Distance
Stopping Sight Distance 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 obstacle.
For crest vertical curves, AASHTO defines the minimum length L min,
feet (m), of crest vertical curves based on a required sight distance S, feet (m), as that given by
1) For S
Lmin=(AS2)/[100 (?2H1 +?2H2)2]
2)For S>L
Lmin=25-[200((?H1+?H2)2]/AS2
where
A= algebraic difference in percent, grades 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 situated in terms of the rate of vertical curvature K defined by
K=L/A
where
L= length, ft (m), of vertical curve
. K is usable 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 impenitence gradient.