Sight distance

1. Sight Distance
By:
Ahmed Abbas Saleem

2. Stopping Sight Distance SSD
 Length of roadway ahead visible to the
driver
Note:
The minimum designed stopping sight
distance should be long enough for a driver
going at design speed to see an object
(potential hazard) and stop before hitting the
object

3. Minimum Required Stopping
Sight Distance
Two components:
 Distance traveled while reacting

(2.5 seconds assumed reaction time)
 Distance traveled while braking

Assumes wet road (decel rate of 3.4 m/sec2
or
11.2 ft/sec2
)
 Can be calculated; however, minimum is
usually obtained by HDM, chapter 2 or
AASHTO book

4. Minimum Design SSD; 2001
AASHTO

5. Minimum Design SSD on grades

6. During Design
 Determine minimum SSD
 Calculate actual SSD/HSD and check
that it meets the minimum
 SSD-actual stopping sight distance
(crest)
 HSD-headlight sight distance (sag)

7. Vertical Curves
 Crest Curves
 Sag Curves
 Careful with signs of G1 and G2!!

8. Crest Vertical Curve
 Height of Eye: 1070 mm; 3.5 ft
 Height of Object: 600 mm; 2.0 ft
 (for passing HO=1070 mm; 3.5 ft)
 G1 and G2-grades (%)
 L=length of vertical curve (ft or m)
 S=sight distance (ft or m)

10. Equations-Crest Curves

11. Crest Curve Example
English, Solve for L
 G1=+3.9% and G2=+1.1%
 PVI Sta=20+50; Elev=1005.00’
 Determine the minimum length of crest
vertical curve for a design speed of 50
mph

13. Crest Curve Example
 Minimum SSD is 425’ (see previous slide)
 Assume S<=L
 G1-G2=2.8
 L=234’ (Check S<L; no)
 Assume S>L
 L=80’ (Check S>L; yes)

14. Sag Vertical Curve
 Headlight Height: 600 mm; 2 ft
 Headlight Divergence of 1 degree upwards
 G1 and G2-grades (%)
 L=length of vertical curve (ft or m)
 S=sight distance (ft or m)

16. Equations-Sag Curves

17. Sag Curve Example
Metric; Solve for L
 G1=+1.86% and G2=+5.04%
 L=300m
Find HSD
 Assume S>L
 S=375m (S>L; ok)
 Note: S<L; quadratic equation

18. Sight Distance on Horizontal
Curves
 Sight distance can also be a problem
on horizontal curves (buildings,
embankments, tree growth, etc.)
 The line of sight is a chord of the curve.
The sight distance should be measured
along the centerline of the inside lane of
the curve (not the centerline of the
roadway)

19. Sight Distance on Horizontal
Curves

20. Equations
M (HSO) = Horizontal sight line offset, (m) or (ft)
S = Stopping sight distance, (m) or (ft)
R = Radius of curves, (m) or (ft)

22. At intersections where two or more roads meet,
visibility should be provided for the drivers approaching
the intersection from either sides.
They should be able to perceive a hazard and stop
the vehicle if required. Stopping sight distance for each
road can be computed from the design speed.
The sight distance should be provided such that the
drivers on either side should be able to see each other.
This is illustrated in the figure .
Sight distance at intersections

23. Design of sight distance at intersections may be used on three possible conditions:
•Enabling approaching vehicle to change the speed
•Enabling approaching vehicle to stop
•Enabling stopped vehicle to cross a main road