Topic 6 - Sight Distance
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Transcript of Topic 6 - Sight Distance
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
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)
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.
Stopping Sight Distance
2
2 1.075V
da
=
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
Stopping Sight Distance
2
2
3032.2
Vd
aG
=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)
Stopping Sight Distance
2
2
3032.2
Vd
aG
=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
2
60 3600444.4
11.2 30 0.35 0.0830 0.0832.2
d ft= = =æ ö -÷ç - ÷ç ÷çè ø
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
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: 10.2 - 11.2 s
Speed/Direction Change on Urban Highway: 14.0 – 14.5 s
Other Areas:
Interchanges
Toll PlazaNote: In some instances, DSD may not be economically feasible. In these cases, the use of suitable traffic control devices should be provided
0
10
20
30
40
50
60
70
0 200 400 600 800 1000 1200 1400
Distance (ft)
Sp
eed
(m
ph
)
T=2.5 s T=10.2 s
Passing Sight Distance
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
Definition: The sight distance needed for allowing a faster vehicle to pass a slower vehicle on a two-lane highway
Passing Sight Distance
Initial Maneuver Distance
Occupying Left-Lane Distance
Clearance Distance Opposing Vehicle Distance
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
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)
To
tal
Dis
tan
ce (
ft)
Passing Sight Distance
ti=3.6; t2=9.3
ti=4.5; t2=9.3
ti=4.5; t2=10.5
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
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
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
Initial Maneuver Distance (d1):
1 1.472i
i
atd 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 seconds
Average acceleration a is between 1.4 and 1.5 ft/s2
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
Clearance Distance (d3):
Passing Sight Distance
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