12/11/2009
Road features and safety
•
•
•
•
•
•
•
•
•
Road alignment
horizontal (bends)
Vertical (slopes)
Road or lane width
Shoulder surface and width
Road surface
R d environment
Road
i
t
Lateral obstacles
Signing
Central and lateral markings
Junction design
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12/11/2009
Summary
• I. Part played by infrastructure factors in
accident
id t processes
• II. Relationships between road
characteristics and accidents
• III. What rationale for improving roads ?
Part played by infrastructure
factors in accident processes
•
•
•
•
I.1 in the normal traffic situation
I.2 in the pre-crash situation
I.3 in the emergency situation
I.4 in the crash situation
BREAKING
POINT
CRASH
SITUATION
D. A. I.
NORMAL
DRIVING
EMERGENCY
SITUATION
POST-CRASH
SITUATION
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I.1 in the normal traffic situation
• The road user activity:
- collecting information from the road and its
environment
- treating the information in order to build up
a representation of the driving situation
- controling speed and trajectory of the
vehicle
• The road features must help drivers
- To exert this control adequately
- To foresee any hazard that may occur
Road more « legible » = better safety
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Exemples
Douala-Yaoundé (Cameroun)
Dr Joëlle TAMBEKOU, RSD, Cameroun
Dr Emmanuel Lagarde, INSERM, France
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I.2 in the pre-crash situation
• The pre-crash situation is a breaking point
i a road
in
d user movementt
- Signing and marking indicate to the roadusers the hazards they need to expect
- Road features should also help reduce
potential encounters and therefore
conflicts between road-users
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Exemples
I.3 in the emergency situation
• The road must « forgive »
• In
I particular,
ti l shoulders
h ld
should
h ld provide
id sufficient
ffi i t
space:
- to make it possible to avoid frontal collision
- to return to normal trajectory after a car has
encroached
- to be used by non-motorized traffic (pre-crash
situation)
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12/11/2009
I.4 in the crash situation
•
•
The trajectory may end by:
a lateral
l t l or ffrontal
t l collision
lli i with
ith obstacles
b t l
a rollover
Through secondary safety measures , reduce
the severity of injuries by
- clearing the road sides
- removing hard obstacles
- protecting vehicles against them through
restraint systems (guardrails or barriers)
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12/11/2009
II. Relationships between road
characteristics and accidents
• Quantify the relationships
• Identify possible remedial measures for
the three road features on rural roads
- width of the carriageway and of shoulders
- horizontal alignment
- obstacles on the road sides
• The study of the relationships between
road
d characteristics
h
t i ti and
d accidents
id t should
h ld
help:
- to correct dangerous road sections
- to define standards and safety rules
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12/11/2009
II.1 width of the carriageway
and of shoulders
• Accident rates decrease with increases in
l
lane
and
d shoulder
h ld width
idth
• Optimal carriageway and shoulder width
depends upon traffic volumes and road
alignment
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12/11/2009
II.2 horizontal alignment
• There is a strong link between radius of curves
and accidents
accidents, short radius being more
hazardous
• At curves with a large radius (over 150 m.),
safety is negatively affected by:
- approaches to curve designed for speed too
high
- poor superelevation in curve
- variation in the curvature radius
- lack of marking, no clear road sides
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12/11/2009
• Putting in specific chevron signs as well as
markings
ki
are costt effective
ff ti
countermeasures
• If larger financial means are available, it is
recommanded to flatten the curve
(increase the radius)
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12/11/2009
II.3 obstacles on the road sides
• When coupled with near-by roadside hazards,
encroachements can result in
- single vehicle against a fixed object
- rollover accident
• Severity of impact vary with
- the differnt kinds of objects likely to be hit : trees,
buildings, fences, signposts, lamp-posts, utility
poles, drainage facilities, ditches
- the size and mass of the vehicle
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12/11/2009
• Exits from carriageway are described by
t
two
features:
f t
- exit angle (in average 6 degrees)
- exit attitude (in line with the previous
trajectory or sideways)
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12/11/2009
• In general, maximizing clear-zone width
(up to ten metres) is the best solution to
reduce accident severity
- Mounting fragilized lamp-poles, utility
poles
- relocating signs and poles
- installing gardrails
improve secondary safety
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12/11/2009
III. What rationale for improving
roads ?
• When accident risk is high at some road
stretchs,
t t h undertake
d t k corrective
ti action,
ti
whatever the traffic volumes
• When accident risk is moderate on parts of
the road network with high traffic volumes,
general upgrading should be considered
• The retro-active influence of road
improvement on safety and risk should be
controlled of accident
• Better roads induce higher speed which, in
turn, lead to higher numbers of accidents
• Any safety measure on roads and their
environment
i
t should
h ld th
thus b
be completed
l t db
by
some educative action addressing the
road-users concerned
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