Sight Distance and Stopping Sight Distance in Traffic Engineering

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Lecture. Dr. Sady Abd Tayh
 
Civil Engineering
Al-Mustansiriyah University
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References:
References:
 
Nicholas J. Garber, Lester A. Hoel,  (2010) Traffic and
Highway Engineering, 4
th
 ed.
Roger P. Roess
,  
William R. McShane
,  
(2004), update
copy (2016) Traffic Engineering (4th Edition).
Highway Capacity Manual, (HCM, 2010).
Traffic Engineering Handbook: Institute of
Transportation Engineers, Seventh Editions (2016).
Sight Distance:
Sight Distance:
 
is the length of the roadway
ahead visible to the driver.
 Safe highways must be designed to give
 Safe highways must be designed to give
drivers a sufficient distance of clear vision a
drivers a sufficient distance of clear vision a
head so that they can avoid hitting
head so that they can avoid hitting
unexpected obstacles and can pass slow
unexpected obstacles and can pass slow
moving vehicle.
moving vehicle.
Stopping Sight Distance (SSD):
Stopping Sight Distance (SSD):
Is the minimum distance required to stop a vehicle
Is the minimum distance required to stop a vehicle
traveling at the design speed before it reach's a
traveling at the design speed before it reach's a
stationary object in the vehicles path.
stationary object in the vehicles path.
 
 
The stationary object may be another vehicle or
The stationary object may be another vehicle or
some object within the roadway.
some object within the roadway.
Stopping Sight Distance: is the sum of two
Stopping Sight Distance: is the sum of two
distances:
distances:
1.
Brake reaction distance.
Brake reaction distance.
2.
Braking distance.
Braking distance.
     
Intervals between the driver recognizes the object or hazard a head
Intervals between the driver recognizes the object or hazard a head
and the instant the brakes are actually applied.
and the instant the brakes are actually applied.
     
A reaction time (2.5) sec. can be considered adequate, some drivers
takes 3.5 seconds to response, generally (1.5-2.5) sec.
Where:
v: vehicle velocity (km/hr).
t: time (sec.).
s: slope of the road (in decimal form).
SSD:  stopping sight distance (m).
f: coefficient of friction between tires and roadway.
 
 
Presence of moisture, snow,mud.
Type and condition of pavement surface.
Treat of tires.
Usually taken from table of wet pavement
condition.
   
Solution:
Solution:
A)
A)
          
SSD
= 26.48 m.
Hence, the driver will be able to stop in time.
B)
B)
          SSD
= 42 m.
Hence, the driver will not be able to stop in time.
 
 
Passing Sight Distance (PSD):
Passing Sight Distance (PSD):
On most two lane two way highways, vehicles
On most two lane two way highways, vehicles
frequently overtake slower- moving vehicles by
frequently overtake slower- moving vehicles by
using the lane of opposing traffic. For safe
using the lane of opposing traffic. For safe
maneuver passing, the driver should be able to see a
maneuver passing, the driver should be able to see a
sufficient distance a head.
sufficient distance a head.
 
 
The minimum passing sight distance for two-lane
The minimum passing sight distance for two-lane
highway is the sum of the four distance shown in
highway is the sum of the four distance shown in
figure below.
figure below.
 
 d1:  
 d1:  
distance traversed during perception –reaction time and during initial
distance traversed during perception –reaction time and during initial
acceleration to the point where the passing vehicle just enters the left lane.
acceleration to the point where the passing vehicle just enters the left lane.
d
d
2
2
:  
:  
distance  traveled  the time the passing vehicle is traveling in the left
distance  traveled  the time the passing vehicle is traveling in the left
lane.
lane.
d
d
3
3
:  
:  
distance between
distance between
 the passing vehicle  and the opposing  
 the passing vehicle  and the opposing  
vehicle at the
vehicle at the
end of the passing maneuver.
end of the passing maneuver.
d
d
4
4
:  
:  
distance  moved by the opposing vehicle during two thirds of the time
distance  moved by the opposing vehicle during two thirds of the time
the passing vehicle is in the left lane ( usually taken to
the passing vehicle is in the left lane ( usually taken to
 be 2/3 d2 ).
 be 2/3 d2 ).
t1 
t1 
: 
: 
time for initial maneuver (sec.)
time for initial maneuver (sec.)
a
a
  : 
  : 
average acceleration rate (mi/h/sec.)
average acceleration rate (mi/h/sec.)
v
v
  : 
  : 
average speed of passing vehicle (mph).
average speed of passing vehicle (mph).
m
m
: 
: 
difference in speeds of passing and impeder
difference in speeds of passing and impeder
vehicles.
vehicles.
 
t1 
t1 
: 
: 
time passing vehicle is traveling in left lane (sec.)
time passing vehicle is traveling in left lane (sec.)
v
v
  : 
  : 
average speed of passing vehicle (mph).
average speed of passing vehicle (mph).
 
 
 
 
    The clearance distance (d3) between the passing vehicle
    The clearance distance (d3) between the passing vehicle
and the opposing vehicle at the completion of passing
and the opposing vehicle at the completion of passing
maneuver has been found to vary between (100-300)ft.
maneuver has been found to vary between (100-300)ft.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Sight distance is crucial for drivers to spot obstacles and pass safely on highways. Stopping Sight Distance (SSD) is the minimum distance required for a vehicle to stop before reaching a stationary object. Factors affecting SSD include reaction time, speed, and road conditions. Passing Sight Distance (PSD) is essential for overtaking safely on two-lane highways. Engineers consider these distances to ensure road safety and design effective traffic management systems.

  • Traffic Engineering
  • Sight Distance
  • Stopping Sight Distance
  • Road Safety
  • Highway Design

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  1. 3rdStage Lecture 12 Lecture. Dr. RanaAmir Yousif Lecture. Dr. SadyAbd Tayh Civil Engineering Al-Mustansiriyah University 2019-2020

  2. References: Nicholas J. Garber, Lester A. Hoel, (2010) Traffic and Highway Engineering, 4thed. Roger P. Roess, William R. McShane, (2004), update copy (2016) Traffic Engineering (4th Edition). Highway Capacity Manual, (HCM, 2010). Traffic Engineering Handbook: Transportation Engineers, Seventh Editions (2016). Institute of

  3. Sight Distance: is the length of the roadway ahead visible to the driver. Safe highways must be designed to give drivers a sufficient distance of clear vision a head so that they unexpected obstacles and can pass slow moving vehicle. can avoid hitting

  4. Stopping Sight Distance (SSD): Is the minimum distance required to stop a vehicle traveling at the design speed before it reach's a stationary object in the vehicles path. The stationary object may be another vehicle or some object within the roadway. Stopping Sight Distance: is the sum of two distances: Brake reaction distance. Braking distance. 1. 2.

  5. Intervals between the driver recognizes the object or hazard a head and the instant the brakes are actually applied. A reaction time (2.5) sec. can be considered adequate, some drivers takes 3.5 seconds to response, generally (1.5-2.5) sec. Where: v: vehicle velocity (km/hr). t: time (sec.). s: slope of the road (in decimal form). SSD: stopping sight distance (m). f: coefficient of friction between tires and roadway.

  6. Presence of moisture, snow,mud. Type and condition of pavement surface. Treat of tires. Usually taken from table of wet pavement condition.

  7. Solution: A) SSD= 26.48 m. Hence, the driver will be able to stop in time. B) SSD= 42 m. Hence, the driver will not be able to stop in time.

  8. Passing Sight Distance (PSD): On most two lane two way highways, vehicles frequently overtake slower- moving vehicles by using the lane of opposing traffic. For safe maneuver passing, the driver should be able to see a sufficient distance a head. The minimum passing sight distance for two-lane highway is the sum of the four distance shown in figure below.

  9. d1: distance traversed during perception reaction time and during initial acceleration to the point where the passing vehicle just enters the left lane. d2: distance traveled the time the passing vehicle is traveling in the left lane. d3: distance between the passing vehicle and the opposing vehicle at the end of the passing maneuver. d4: distance moved by the opposing vehicle during two thirds of the time the passing vehicle is in the left lane ( usually taken to be 2/3 d2 ).

  10. t1 : time for initial maneuver (sec.) a : average acceleration rate (mi/h/sec.) v : average speed of passing vehicle (mph). m: difference in speeds of passing and impeder vehicles.

  11. t1 : time passing vehicle is traveling in left lane (sec.) v : average speed of passing vehicle (mph). The clearance distance (d3) between the passing vehicle and the opposing vehicle at the completion of passing maneuver has been found to vary between (100-300)ft.

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