1. SIGHT DISTANCE
Spring 2015

2. Stopping Sight Distance Decision Sight Distance
Passing Sight Distance
Note: A driver’s ability to see ahead is of the utmost importance in the safe and efficient operation of a vehicle on a highway

3. Example: Sight Distance

4. Stopping Sight Distance
Definition: The available sight distance on a highway that allows a vehicle traveling near the design speed to stop before reaching a stationary object in its path
Brake Reaction Time (d1)
Braking Distance (d2)

5. Stopping Sight Distance
Braking Reaction Time Distance:
Where,
d1 = BRT distance (ft)
V = Design Speed (mph)
t = Braking reaction time (s)
Note: Usually 2.5 seconds is used as the BRT. However, some drivers can take as long as 3.5 seconds. The absolute minimum should be 1.64 seconds.

6. Stopping Sight Distance
Braking Distance:
Where,
d2 = Braking distance (ft)
V = Design Speed (mph)
a = deceleration rate (ft/s2)
Note:
Deceleration rate for design: 11.2 ft/s2
Deceleration rate in emergency: 14.8 ft/s2

7. Stopping Sight Distance
Braking Distance (with Grade):
Where,
d2 = Braking distance (ft)
V = Design Speed (mph)
a = deceleration rate (ft/s2)
G = Grade in percent (- for downgrade, + for upgrade)

8. Stopping Sight Distance
Example: Calculate the braking distance for a highway section characterized with a design speed of 60 mph and a 8% downgrade

10. Stopping Sight Distance: Design Values

11. Stopping Sight Distance: Design Values

12. Decision Sight Distance
Definition: The available sight distance on a highway needed to detect an unexpected or otherwise difficult-to-perceive information in a roadway environment
Decision Time (d1)
Braking Distance (d2)
Same as SSD

13. Decision Sight Distance
General Guidelines:
Stop on Rural Highway: 3.0 s
Stop on Urban Highway: 9.1 s
Speed/Direction Change on Rural Highway: s
Speed/Direction Change on Urban Highway: – 14.5 s
Other Areas:
Interchanges
Toll Plaza
Note: In some instances, DSD may not be economically feasible. In these cases, the use of suitable traffic control devices should be provided

14. Decision Sight Distance: Design Values

15. T=10.2 s
T=2.5 s

16. Passing Sight Distance
Definition: The sight distance needed for allowing a faster vehicle to pass a slower vehicle on a two-lane highway
Sum of four distances:
d1 – Distance traversed during the perception and reaction time + acceleration to the point of encroachment
d2 – Distance traveled while the passing vehicle occupies the left lane
d3 – Distance between the passing vehicle at the end of its maneuver and the opposite vehicle
d4 – Distance traversed by an opposing vehicle for 2/3 of the time the passing vehicle occupies the left lane

17. Passing Sight Distance
Initial Maneuver Distance
Clearance Distance
Opposing Vehicle Distance
Occupying Left-Lane Distance

18. PASSING SIGHT DISTANCE
Important assumptions
The overtaken vehicle travels at uniform speed
The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section
The passing vehicle accelerates during the maneuver and its average speed is 10 mph higher than the overtaken vehicle
When the passing vehicle returns to its lane, there is suitable clearance length between it and the oncoming vehicle in the other lane

19. Passing Sight Distance: Design Values

20. PASSING SIGHT DISTANCE10 30 50 70
Mph

21. PASSING SIGHT DISTANCE
Based on HSM – Multiple Passing Zones

22. Passing Sight Distance
ti=4.5; t2=10.5
ti=4.5; t2=9.3
ti=3.6; t2=9.3

23. PASSING SIGHT DISTANCE
Important design consideration
The range of speeds are affected by traffic volume
Low volume – few vehicles need to be passed
High volume – few opportunities to pass
Should provide as many as passing sections as possible
Should not be confused with other warrants used for designing no passing zones

24. PASSING SIGHT DISTANCE
Important design consideration (cont’d)
Overtaking a vehicle is easier for downgrade segments
Need to provide longer passing sight distance for upgrade
In mountainous terrain, it may be more economical to provide for a “climbing lane”
The number of passing opportunities greatly affects the capacity of your facility

25. Criteria for Measuring Sight Distance
Driver Eye Height
Passenger Car: 3.5 ft
Large Trucks: 5.9 to 7.9 ft
SSD Object
2.0 ft
PSD Object
3.5 ft
Object

26. Measuring Sight Distance

27. Passing Sight Distance
Previous Detailed Calculations

28. Passing Sight Distance
Initial Maneuver Distance (d1):
Where,
ti = time of initial maneuver (s)
v = average speed of passing vehicle (mph)
a = average acceleration rate (ft/s2)
m = difference in speed between passing and passed vehicles (mph)
Note: Average time for ti is between 3.6 and 4.6 seconds
Average acceleration a is between 1.4 and 1.5 ft/s2

29. Passing Sight Distance
Distance for Occupying Left Lane (d2):
Where,
v = average speed of passing vehicle (mph)
t2 = time passing vehicle occupies the left lane (s)
Note: Average time for t2 is between 9.3 and 10.4 seconds

30. Passing Sight Distance
Clearance Distance (d3):
Usually varies between 100 to 300 ft
Opposite Vehicle Distance (d4):
Assume to be 2d2/3
Note: Opposite vehicle assumed to travel at the same speed as the passing vehicle

31. Passing Sight Distance: Design Values

32. Passing Sight Distance: Design Values