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Microsoft PowerPoint - Lecture 9 - CIV2701-Grades and Sight Distances

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Microsoft PowerPoint - Lecture 9 - CIV2701-Grades and Sight Distances Powered By Docstoc
					      T h e   U n i v e r s i t y       o f    S o u t h e r n     Q u e e n s l a n d
                                                                      www.usq.edu.au




     Grades and Sight Distances




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      Grades

Expressed as % or ratio eg 1 in 20

When determining grade limitations consider:
• Poor performing vehicles – commercial
  vehicles, cars with caravans, low powered
  vehicles – this sets limit for maximum grades
• Quality of service – flatter grades are less
  demanding on vehicle and driver
• Speed differences between different types of
  vehicles ie overtaking, queuing – leads to safety
  problems
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 Grades
Uphill Speed of Vehicles
• Max grade for heavy vehicles 15%

Downhill Speed of Vehicles
• Cars generally have efficient braking system
  and generally not affected by downhill
• Safety aspect of downhill is a concern with
  regard to high speeds caused by equipment
  failure




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 Effect of Grade
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     Maximum and Minimum Grades
  • Minimum longitudinal grade >0.3%
  • Care with drainage if < 1%
  • 3% satisfactory on high speed roads for most
    vehicles
  • <6% lower design speeds possible
  • >10% creates problems – slow climbing and
    fast down – need to create climbing lanes




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     Steeper Grades

Steeper grades can be justified if:
• Number of heavy vehicles is low
• Difficult terrain
• On less important roads

Ascending traffic – increase grade should not
  decrease speed by >20%
Descending traffic – shouldn’t require excessive
  breaking
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   Steeper Grades


   • 12-15% used on low volume roads
   • Up to 30% use where no commercial vehicles
     and only on short lengths
   • Individual geometric speed elements may be
     increased towards the foot of a steep grade if
     warranted




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   Sight Distance
Sight Distance – is the distance of the drivers
visibility along the road.

Distance along carriageway between driver and an
object or between two objects at a specific height.

Ability to perceive an object reduced when travelling
around curves, accelerating or braking.

Reaction time decreased by fatigue and influence of
drugs.
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Sight Distance




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       Sight Distance
Constraints Assumed in Determining Sight Distance

• Total Reaction Time – time between when hazard comes into
view and instant vehicle begins to slow – in road design use 2.5
seconds but may use 2.0 or 1.5 seconds in low speed
environments
• Height of Driver – adopt 1.05m for passenger vehicles and 1.8 for
commercial and 2.4m for trucks
• Height of Stationary Object – 0.2m assumed for most, but may
assume 0.0m for line markings
• Height of Approaching Vehicle – adopt 1.05m
• Height of Vehicle Tail Stop Light – adopt 0.6m
• Height of Headlight – used for sag curves and adopt 0.75m




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       Sight Distance
  Categories of Sight Distance

  •   Stopping Sight Distance
  •   Intermediate (combined) Sight Distance
  •   Overtaking Sight Distance
  •   Headlight Sight Distance
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   Sight Distance


   Stopping Sight Distance – is
   distance travelling at design speed
   needs to stop before reaching a
   stationary object.

   It has two components
   • distance travelled during reaction
   time
   • breaking distance




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   Sight Distance
Reaction Distance – can be computed by

        d = tv
Where             d = distance in metres
                  t = time in seconds
                  v = speed in m/sec

If we convert v (m/s)to V (km/hr) and adopt reaction
time of 2.5 sec then
       d = 0.7V metres
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            Sight Distance
Braking Distance – can be computed by

         d1 = v2/2a
Where    d1 = distance in metres
                   a = rate of deceleration in m/s2
                   v = speed in m/sec
Now                F = Wa/g
So                 F/W= fL
Where    F = braking force
                  W= weight of vehicle
                   fL = coefficient of longitudinal friction
                  g = acceleration due to gravity (9.8 m/s2)
So therefore      a = fLg
                  d1 = v2/(2 fLg) then converting v to km/hr (V)
                   d1 = V2/(254 fL)
So Stopping Sight Distance (SSD) = d + d1
                            SSD = 0.7V + V2/(254 fL )




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            Sight Distance
         Tables for fL = coefficient of
         longitudinal friction
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      Example – you to do
  You are travelling in an urban area in your Holden V8
  ute. As per usual you are day dreaming about the
  weekend and going to the B &S and picnic races out at
  Injune. You have just put on a new bullbar for the trip.

  A small dog runs out on the road about 90m in front of
  you. You are doing 74km/hr (in a 60 zone) and as you
  are young and fit your reaction time is estimated to be
  1.7 seconds.

  What happens to the little puppy dog?




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      Sight Distance
Effect of Grade on Braking Distance

Longer breaking distances are required for vehicles travelling
down a grade and shorter for upgrades.

The breaking distance formula can be amended to take this into
account, so       d1 = V2/254 (fL +/- 0.01G)
Where G is the longitudinal grade expressed as a % (+ve for
uphill and –ve for downhill)
So Stopping Sight Distance on grades is
                              SSD = 0.7V + V2/{254 (fL +/- 0.01G)}
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        Example – you to do
You are travelling in an urban area in your Holden V8 ute. As
per usual you are day dreaming about the weekend and going
to the B &S and picnic races out at Injune. You have just put
on a new bullbar for the trip.

A small dog runs out on the road about 90m in front of you.
You are doing 74km/hr (in a 60 zone) and as you are young and
fit your reaction time is estimated to be 1.7 seconds.

I forgot to say that you were travelling down a hill with a grade
of 9%.

What happens to the little puppy dog, NOW?




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        Poor Puppy
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   Use of Tables for Calculating Sight
   Stopping Distance (SSD)




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   Intermediate Sight Distance
Intermediate Sight Distance – is often referred to as
combined distance travelling at design speed needs
to stop before reaching a stationary object.

It allows two drivers travelling towards each other at
design speed to stop before meeting and allows
overtaking with safety when no opposing traffic in
view.

On a crest curve makes it possible to see a 1.05m
high object at twice the stopping sight distance and a
0.2m object approx 1.4 times stopping sight distance.
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   Overtaking Sight Distance
Overtaking Sight Distance – distance measured at
1.05m between two vehicles to allow safe overtaking.

Consists of sum of:
• distance travelled during perception,
• distance travelled during overtaking
• distance travelled by an oncoming vehicle

Variables to be considered include speeds of all
vehicles (overtaking, passed, oncoming), judgement
of driver, type of vehicle, risk




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    Overtaking Sight Distance Components


Austroads have adopted three considerations when
establishing overtaking sight distances, namely:
• Establishment – minimum distance to enable a given
proportion of drivers to commence an overtaking manoeuvre
– called Establishment Sight Distance (ESD)
• Continuation – a critical sight distance to enable a driver,
after the ESD has been established, to safely complete or
abandon manoeuvre – called Continuation Sight Distance
(CSD)
• Re-establishment – frequency at which the ESD is re-
established. On high volume roads a vehicle might be
required to trail another. A re-establishment distance is the
time spent trailing a vehicle before attempting to overtake.




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    New Definition of a Manoeuvre from
    Austroads
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   Overtaking Sight Distances Tables




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   Overtaking Sight Distance Assumptions


Overtaking distance is calculated assuming:
• Vehicle being overtaking is travelling less than
  design speed
• Overtaking and oncoming vehicles travelling at
  design speed
• Overtaking is done by accelerating throughout
  and completed when vehicle returns to lane
Cost for constructing for overtaking can be
  expensive so to need to consider design
  carefully.
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    Headlight Sight Distance
Headlight Sight Distance – ensures that the
roadway ahead is illuminated to at least the stopping
distance.

Assume headlights are 0.75m above road, beam 1
degree upwards and object at 0.0m

Need to consider design in sag curves and combined
with horizontal curves. Headlights may only enable
vision for approx 300m (50m if driving a VW)




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    Restrictions to Visibility


•   Guard rails
•   Bridges
•   Signs
•   Medians
•   Horizontal and vertical curves
•   Vegetation
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Sight Distance
 Application to Two Lane Road

				
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