Page 349 - Civil Engineering Formulas

280 CHAPTER ELEVEN

Stopping sight distance S
Vertical curve length L

H 1 PVC PVI PVT H 2

FIGURE 11.5 Stopping sight distance on a crest vertical curve.

For crest vertical curves, AASHTO defines the minimum length L , ft (m), of
min
crest vertical curves based on a required sight distance S, ft (m), as that given by
AS 2
L min 2 S L (11.18)
100 2 2H 1 2 2H 2

When eye height is 3.5 ft (1.07 m) and object height is 0.5 ft (0.152 m):
AS 2
L min S L (11.19)
1329
Also, for crest vertical curves:
200 H 1 H 2 2
L min 25 S L (11.20)
AS 2
When eye height is 3.5 ft (1.07 m) and object height 0.5 ft (0.152 m):

1329
L min 25 S L (11.21)
AS 2
where A algebraic difference in grades, percent, of the tangents to the verti-
cal curve
H eye height, ft (m), above the pavement
1
H object height, ft (m), above the pavement
2
Design controls for vertical curves can be established in terms of the rate of
vertical curvature K defined by
L
K (11.22)
A
where L length, ft (m), of vertical curve and A is defined earlier. 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.