sight distance
DESCRIPTION
SIGHT DISTANCE. Spring 2014. Sight Distance. 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. Example: Sight Distance. - PowerPoint PPT PresentationTRANSCRIPT
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SIGHT DISTANCE
Spring 2016
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Stopping Sight Distance
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
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Example: Sight Distance
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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)
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Stopping Sight Distance
1 1.47d Vt=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.
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Stopping Sight Distance
2
2 1.075Vda
=
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
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Stopping Sight Distance
2
2
3032.2
Vda G
= æ ö÷ç ± ÷ç ÷çè ø
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)
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Stopping Sight Distance
2
2
3032.2
Vda G
= æ ö÷ç ± ÷ç ÷çè ø
Example: Calculate the braking distance for a highway section characterized with a design speed of 60 mph and a 8% downgrade
( )2
260 3600 444.4
11.2 30 0.35 0.0830 0.0832.2
d ft= = =æ ö -÷ç - ÷ç ÷çè ø
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0
10
20
30
40
50
60
70
80
0 100 200 300 400 500 600 700 800
Distance (ft)
Spee
d (m
ph)
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Stopping Sight Distance: Design Values
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Stopping Sight Distance: Design Values
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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
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Decision Sight Distance
General Guidelines:Stop on Rural Highway: 3.0 sStop on Urban Highway: 9.1 sSpeed/Direction Change on Rural Highway: 10.2 - 11.2 sSpeed/Direction Change on Urban Highway: 14.0 – 14.5 sOther Areas:InterchangesToll PlazaNote: In some instances, DSD may not be economically feasible. In these cases, the use of suitable traffic control devices should be provided
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Decision Sight Distance: Design Values
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0
10
20
30
40
50
60
70
0 200 400 600 800 1000 1200 1400
Distance (ft)
Spee
d (m
ph)
T=2.5 s T=10.2 s
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Passing Sight Distance
Sum of four distances:d1 – Distance traversed during the perception and reaction time + acceleration to the point of encroachmentd2 – Distance traveled while the passing vehicle occupies the left laned3 – Distance between the passing vehicle at the end of its maneuver and the opposite vehicled4 – Distance traversed by an opposing vehicle for 2/3 of the time the passing vehicle occupies the left lane
Definition: The sight distance needed for allowing a faster vehicle to pass a slower vehicle on a two-lane highway
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Passing Sight Distance
Initial Maneuver Distance
Occupying Left-Lane Distance
Clearance Distance Opposing Vehicle Distance
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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
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Passing Sight Distance: Design Values
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PASSING SIGHT DISTANCE
10 30 50 70Mph
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PASSING SIGHT DISTANCE
Based on HSM – Multiple Passing Zones
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0.0
500.0
1000.0
1500.0
2000.0
2500.0
3000.0
30 35 40 45 50 55 60 65 70 75
Passign Speed (mph)
Tota
l Dis
tanc
e (ft
)
Passing Sight Distance
ti=3.6; t2=9.3
ti=4.5; t2=9.3
ti=4.5; t2=10.5
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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
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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
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Criteria for Measuring Sight Distance
Driver Eye Height Passenger Car: 3.5 ftLarge Trucks: 5.9 to 7.9 ft
SSD Object 2.0 ft
PSD Object 3.5 ft
Object
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Measuring Sight Distance
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Passing Sight Distance
Previous Detailed Calculations
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Initial Maneuver Distance (d1):
1 1.472i
iatd t v m
æ ö÷ç= - + ÷ç ÷çè ø
Passing Sight Distance
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 secondsAverage acceleration a is between 1.4 and 1.5 ft/s2
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Distance for Occupying Left Lane (d2):
2 21.47d vt=
Passing Sight Distance
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
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Clearance Distance (d3):
Passing Sight Distance
Usually varies between 100 to 300 ftOpposite Vehicle Distance (d4):
Assume to be 2d2/3Note: Opposite vehicle assumed to travel at the same speed as the passing vehicle
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Passing Sight Distance: Design Values
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Passing Sight Distance: Design Values