Road Safety Training Course for Practitioners:
Problems & Application of 5E’s
Safe Road Design: Curve & Public Place
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ACCIDENT RESEARCH INSTITUTE
What is Curve?
 When a road changes its direction from right to left (vice versa) or
changes its alignment from up to down (vice versa)
Horizontal
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Vertical
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Crashes on Curve
According to National Cooperative Highway Research
Program (NCHRP) 17-18(3) of US Federal Highway
Administration (FHA), more than 25% of fatal crashes
are associated with a horizontal curve.
The average crash rate for horizontal curves is about 3
times that of other types of highway segments.
In Bangladesh, around 8% of total accidents occurred
at bends and the share of casualties for those accidents
are nearly 12%.
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ACCIDENT RESEARCH INSTITUTE
Crashes on Curve: Bangladesh
Straight
Curve
Casualty
Casualty
Acc
Index
Acc
19999
31154
1.56
1771
Casualty
Casualty
Index
3551
2.0
Risk Factor on Curve = 2/1.56 = 1.29
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ACCIDENT RESEARCH INSTITUTE
Crashes on Curve: Bangladesh
Acc. Type
No. of Acc.
% of Total
Acc
No. of
Casualty
HeadO
RearE
90deg
Side
OverT
Hit Obj
ParkV
Ped'n
Animl
Other
Total
478
192
18
144
247
67
36
510
2
75
1769
27.02
10.85
1.02
8.14
13.96
3.79
2.04
28.83
0.11
4.24
100.00
1212
328
31
252
763
146
67
622
5
121
3547
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% of Total Casualty Index
Casualty (Casualty/Acc)
34.17
9.25
0.87
7.10
21.51
4.12
1.89
17.54
0.14
3.41
100.00
2.54
1.71
1.72
1.75
3.09
2.18
1.86
1.22
2.50
1.61
2.01
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ACCIDENT RESEARCH INSTITUTE
Elements of Horizontal Curve
 Design speed
 Radius of curve
 Side friction factor
 Superelevation
 Curve widening
 Visibility and Sight Distance
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Design speed
Design speed is a selected speed used to determine the various
geometric features of the roadway
 It is a design control, rather than a specific design element i.e. It
establishes the range of design values for many of the other
geometric elements of the highway
 It is the expected 85th percentile speed of the motorized traffic
on the new road in the design year.
The selection of a particular design speed is influenced by the
following:
 The functional classification of the highway
 The character of the terrain
 The density and character of adjacent land use
 The traffic volumes expected to use the highway
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Value of Design Speed
For RHD Roads
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For LGED Roads
There is no set values
according to Road
Design Standards: Rural
Roads, 2005
Depending on the road
width, traffic volume
for LGED roads design
speed could be 40 km/h
for plain terrain
Source: Geometric Design Standards for Roads & Highways
Department, 2000
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Radius of curve
 The radius of the horizontal curve is an important design aspect
of the geometric design.
 The maximum comfortable speed on a horizontal curve depends
on the radius of the curve
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Field Measurement of Radius of curve
 Basic ball bank indicator (BBI)
 Advanced BBI,
 Chord length,
 Compass,
 Field survey,
 Global Positioning System (GPS) unit,
 Lateral acceleration,
 Plan sheet,
 Speed advisory plate, and
 Vehicle yaw rate.
https://ceprofs.tamu.edu/mburris/Papers/TRR%201918%20%20COMPARISON%20OF%20RADIUS%20ESTIMATING%20TECHNIQUES%20FOR%20HORIZONTA
L%20CURVES.pdf
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Field Measurement of Radius of curve
A string of known length (usually 100 ft) is stretched
between any two points between the PC and PT of any
curve so that each end just touches the lane edge-line of
the horizontal curve.
An offset distance is measured from the
middle of the string to the lane edgeline.
H
L
R = L2 /8H + H/2
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Field Measurement of Radius of curve
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Side friction factor, f
When vehicles travel over a horizontal curve, the centripetal force keeps the
vehicles from sliding to the outside edge of the curve. In the simplest case, where
the road is not banked, the entire centripetal force is provided by the friction
between the vehicle's tires and the roadway.
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Value of Side Friction Factor, f
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Superelevation
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Design value of superelevation
 The amount of superelevation required for a given radius, speed
and coefficient of friction can be calculated by the following
equation:
Current practice in Bangladesh
 For RHD Roads: between 3% and 7% (Source: Geometric Design
Standards for Roads & Highways Department, 2000)
 For LGED Roads: between 2% and 6.7% (Source: Road Design
Standards: Rural Roads, 2005)
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How super-elevation is attained?
Road Section View
Road Plan View
CL
-2%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-1.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-1%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-0.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
0.0%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
0.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
1%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
1.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
2%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
3%
3%
How super-elevation is attained?
Road Section View
Road Plan View
CL
4%
4%
HowRoad
super-elevation
is attained?
Section View
Road Plan View
CL
3%
3%
How super-elevation is attained?
Road Section View
Road Plan View
CL
2%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
1.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
1%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
0.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
0.0%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-0.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-1%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-1.5%
2%
How super-elevation is attained?
Road Section View
Road Plan View
CL
-2%
2%
Lp
Le
Lc
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If Shift, S =Lp2/24R > 0.25
Placement of Transition required
Lp = Le – 0.4 V
Source: Road Planning and Design Manual, South Queensland,
Australia,
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Curve widening
Curve widening
refers to the
additional width of
carriageway that is
required on a curved
section of a road
over and above that
required on a
straight alignment
In general extra
width is provided on
the horizontal
curves when the
radius is less than
300 m.
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Calculation of Widening
 the radius of the curve
 width of lane on a straight road
 vehicle length and width
 vehicle clearance
 According to AASHTO, the curve widening is calculated from the
equation below.

 w = W c – Wn
 where,


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w
Wc
Wn
= widening of traveled way on curve
= width of the traveled way on curve
= width of the traveled way on tangent
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Visibility and Sight Distance
 Minimum sight distance (for safety) should be equal to the safe
stopping distance
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Determining the Curve Parameters
1.
2.
3.
4.
5.
Decide what sight distance to use
Determine the minimum curve radius
Determine the minimum superelevation requirements
Determine minimum transition lengths
Determine whether there are any curve widening requirements
1.) Determine a reasonable maximum superelevation rate.
2.) Decide upon a maximum side-friction factor.
3.) Calculate the minimum radius for your horizontal curve.
4.) Iterate and test several different radii until you are satisfied with
your design.
5.) Make sure that the stopping sight distance is provided
throughout the length of your curve. Adjust your design if
necessary.
6.) Design the transition segments.
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Major Deficiencies on Curve
 Lack of Sign, marking and
delineation devices
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Major Deficiencies on Curve
 Vision obstruction or inadequate sight distance due to plantation, road
side infrastructure development
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Major Deficiencies on Curve
 Side road/access road at bend
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Major Deficiencies on Curve
 Lack of Super elevation
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Major Deficiencies on Curve
 Lack of Extra widening
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Basic Treatment Options
Road Marking:
 Centerline (F6)
 Edge line (F9)
 Special Speed Limit Marking
(F18)
 Curve Ahead Marking
Delineation:
 Delineators/Guidepost ((B56)
 Reflector on Road Side Object
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Traffic Sign:
•Bend at left / right (B10),
•Advisory Speed Limit Sign (A26),
•Combination Horizontal
Alignment/Advisory Speed (B10 and D12)
sign
•No overtaking Sign (A20)
•Chevron Alignment (B55) sign
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Roadway Geometric Improvements
 Superelevation
 Curve Extra Widening
 Driver alerting Devices/Rumble strips
 Shoulder Drop-Off Elimination
 Widen Shoulder
 Straightening
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Bus Stop/Bus Lay-by
 Buses and other vehicles use the existing carriageway and
shoulder for passenger pick-up/drop-off activities which hinders
the through movement of traffic and increases the potentiality of
conflict.
 This activity also exposes passengers in a more vulnerable
situation as they are forced to board/alight on-street.
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Bus Stop/Bus Lay-by
Where to use
 on straight sections
 Haat/bazaar/market places
 - Educational institutions
 - Hospitals/medical centers
 - Any public places of importance
Where not to use
 Inside of curves
 Within 15 m of junction approach
 Just prior to the Bridge approach
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Bus Stop/Bus Lay-by
 Research has shown that on roads with speeds of 40 mph (60 km/h)
and above, the ideal layout would comprise the following bus bay
arrangement (HCC, 2004):
 A 10º entry taper (14.2m to 15.6m)
 A 24m long parallel section
 A 14º exit taper (between 10.8 and 12m)
 The taper lengths are dependent upon bay width, W, so the overall
length of a bus bay (L) is given by the formula below:
 L = 24 + W (1/tan 10º + 1/tan14º)
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Bus Stop/Bus Lay-by
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Haat/Bazaar/Market
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Haat/Bazaar/Market
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Road adjacent School
 Due to lack of sufficient/designated passenger drop-off area child







passengers are forced to board/alight on-street which exposes
them in a vulnerable situation. The situation might be worsening
at time of starting and ending of school hours.
Child pedestrian are forced to use road surface due to insufficient
shoulder width and ill maintained shoulder
Visibility of children might be obstructed by on-street parked
vehicles
Often children are exposed on road surface while walking in group
Lack of crossing facilities for school children
Lack of Sign/marking
Visibility of signage from approaching traffic is not adequate
Inconsistency in sign
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Glimpses of Problems
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Supplementary Plate D3
B 25
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Thank You All
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