1. Bloody Routes
By
Mona Khafagy, MSc., P.E.
Highway and Transportation Expert
Research and Teaching Associate
American University in Cairo
2016

2. Effect of Defects in Road Geometric
Elements on Accident Rates
Contents
Carriageway
Horizontal Curve
Vertical Curve
Combined Horizontal and Vertical Curves
Shoulders and Medians
Conclusion

3. CAUSES OF ACCIDENTS
The Handbook of Highway Engineering, Australia,2006
 Surface Condition
 Road Geometry
Road Side Features
Road Risk Factors
First edition of AASHTO Highway
Safety Manual was published in 2010.

4. Geometric Design Elements
Number of Lanes (Lane)
Lane Width (Foot/m)
Surface Condition (Good/Bad)
Grade on Tangent (%)
Grade on Curve (%)
Sight Distance (Feet/m)
Radius of curve
Degree of curvature
Superelevation
Shoulder
Width (Foot/m)
Surface Condition (Good/Bad)
Traffic Control
Delineator (Yes or No) Lighting (Yes or No)
Guide Sign (Yes or No) Marking (Yes or No)
Median
Median Width (Foot/m)
Vertical Curve AlignmentHorizontal Curve Alignment
Roadway/ Carriageway

5. Carriageway Characteristics
AASHTO Recommends
• High 1.5 to 2 % (0.015 ft/ft, m/m)
• Intermediate 1.5 to 3 %
Cross Slope describes the slope of a
roadway perpendicular to the centerline. If a
road were completely level, water would
drain off it very slowly
As carriageway width increases,
traffic accidents decreaseSafest Road is a Divided, Multi-lane Road
with full control of access.
The level of safety decreases on three-lane
roads, there is enough space available for
wrong maneuvers and dangerous overtaking
Basic Definitions
AASHTO Recommends lane widths between 2.7 to
3.6 m ( 9 – 12 ft) for safety and comfort of driving.
Lane width should not be less than 2.7 m (9ft) and
used only on low volume ruler roads

6. 2) Poorly Designed Shoulders
42.8% of the fatal collisions reported on highways
occurred on two-lane rural highways.(17)
1) Passing Maneuver
Blind Angles
3) Lights from Opposing Direction
Carriageway Characteristics (Con.)Source of Risk
Why Two Way-Two Lanes Roads are more Dangerous??

7. Every 1% increase in cross-slope results in approximately 32%
increase in rollover crash risk. (3)
20% of truck-involved loss- of-control crashes
and it is likely the Road Cross-sectional
Geometry have played a part in many of
these.(3)
Rollover Crashes
50 % of the severe crashes were rollovers where an
HGV (heavy good vehicles) occupant died. (13)
66% of rollover crashes
occurs while cornering. (3)
Rollover Crashes are more hazardous than other types of crashes.
Carriageway Characteristics (Con.)Source of Risk
How Defects in Cross Slope Affect Accident !!!!

8. Widening of Carriageway
Heavy Vehicle Crash Risk could be REDUCED
BY 8% PER METER OF WIDENING . in New
Zealand (3)
0.25 meters of widening results in Reduction in
Crash Rate By 2% To 2.5% (3)
Widening Lanes from 2.7 to 3.7 m would reduce crash
by 31%. (4)
Widening lanes or shoulders on curves can reduce
curve accidents by as 33 % (12)
Segments with Three Lanes are 40% less in crash risk
than two-lane segments (6)
Carriageway Characteristics (Con.)How to improve!

9. Horizontal Curves
The premier factor in accidents
inside horizontal curves is
RADIUS.
As The Number of Horizontal
Curves per km increases, the total
number of accidents decreases (1)
Drivers Underestimate The Severity
of The Horizontal Curvature.(4)
Accident Severity on Curves is Higher Than
Those Occurring on Straight Roads (6)
Accidents increase when;
Radius is decreased level of curvature is
increased (1)
Accident rate on radii less than 100 m is Four
Times Higher than those with curve radii of
500 m or higher.(5)
Basic Definitions
That is
when

10. Accident Rates in Road Curves are about 1.5 To 4
Times Higher than in Straight Roads (6)
Left-turn curves have higher accident rate than right-
turn on right hand traffic systems.(5)
 A unit increase in Degree of
Curvature is associated with a 5%
Decrease in the crash risk (6)
On horizontal curves, stopping sight distance is
limited and there is a high probability of skidding
 Curve flattening is expected to
reduce accidents by up to 80
percent, depending on the amount of
flattening. (12)
 Adding spiral 'transitions on curves
was associated with a 5% accident
reduction.(12)
Approximately 60%to 66% of all crashes
occurring in horizontal curves are single-vehicle
loss-of-control crashes (off-road crashes).(16)
Horizontal Curve (Con.)Source of Risk
How to improve!

11. SuperElevation
Superelevation: is the amount
of "banking“ of the curve.(9)
The most common cause to truck rollovers is the difficulty
for the driver to assess the combination of Speed, Position
of CoG (center of gravity) and the Cornering Maneuver.
Basic Analysis of Cornering Force
Establish the dynamic equilibrium for steady
cornering, aiming for minimizing steering
effort, weight transfer, tyre wall deformation
and tyre contact patch distortion.(13)
Horizontal Curve (Con.)
Basic Definitions
Superelevation Deficiency (SD): the difference
between the actual Superelevation at a curve and that
prescribed in the American Design Guidelines – the
AASHTO Green Book.(5)
Source of Risk

12. SuperElevation
 Superelevation deficiency of 4% results in
a 4% increase in accident risk. (3)
 Improving deficient superelevation can
reduce accidents by approximately 10%.(12)
 An improvement of 2% in Superelevation
(i.e., increasing Superelevation from 3% to
5% to meet AASHTO design guidelines)
would be expected to yield an accident
reduction of 10 to 11%.(12)
Super-elevation should not exceed 8%. (6)
Horizontal Curve (Con.)
How to improve!

13. The most important risk factors for rollover are
high CoG, high speed, cargo displacement, bad
road conditions, driver behavior (13)
Heavy truck on bad Superelevation (13)
A adjustment of 1 % in super-elevation could
result in a 5 % reduction in heavy vehicle loss-
of-control crash risk while cornering.(13)
Run-Off- the-road accidents were significantly
influenced by Superelevation error.(11)
Superelevation Horizontal Curve (Con.)
How to improve!
Source of Risk
Rollover Crashes on Horizontal Curve

14. How to Improve Safety on Horizontal Curves?
 Reconstruct the curve to make it less sharp
 Widening lanes and shoulders on curves
 Adding spiral transition to curves
 Increasing the amount of Superelevation (when needed)
 Increase the clear roadside recovery distance by reallocating
utility poles and trees
 Assure adequate surface drainage
 Provide increased surface skid resistance on combined
horizontal and downgrade vertical curves
Horizontal Curve (Con.)

15. Vertical Curve
Crest Curve: Stopping Sight Distance
Governs The Design
Sag Curve: Height of Headlight
Beam Governs The Design
Vertical Grade Increases,
Accident Rate Increases
AASHTO indicates that For overall safety, a sag vertical curve
should be long enough that the light beam distance is nearly the
same as the stopping sight distance.
7 feet
100 feet
Basic Definitions

16. Grades of 2.5 to 4% deviation from designed
grade increase crashes by 10% and 20%
respectively. (4)(15)
Accident Rate on down grades is slightly higher
than that on upgrades.(5)
A sharp increase in accident rate on downgrades
greater than 4 % .(5)
Overtaking on upgrades of two lane roads be an
additional hazard.(7)
Crash frequency increases with
gradient percent.(16)
Vertical Curve (Con.)
Source of Risk

17. On sections with high gradient, safety problems may
occur from speed differentials between passenger cars
and heavy vehicles (e.g. heavy vehicles idling on upgrade
sections), as well as vehicles braking on downhill sections
(e.g. increases in braking distances and possibility of
heavy vehicle brake overheating). (16)
Climbing Lanes on rural two-lane roads reduced
the total accident rate by an average of 25%. (15)
A Climbing Lane is an extra lane in the upgrade
direction for use by heavy vehicles whose speeds
are significantly reduced by the grade
Vertical Curve (Con.)
Source of Risk
How to improve!

18. Combined Horizontal and Vertical Curve
The coincidence of a horizontal and a crest vertical curve:
1) Lead to significant limitation of the available sight distance
2) Prevent the prompt perception of the curve
3) Create a false impression of the degree of curvature. (16)
The Deviation from the combined curves
AASHTO specifications had fatal rollover crashes
about 15 times as frequently. (9)
Numbers of accidents are high at points where
high-slope sections end in low-radius horizontal
curves. (1)

19. Combined Horizontal and Vertical Curve (Con.)
An inefficient combination of horizontal and vertical
alignment may lead to road safety problems, even when
the horizontal and the vertical alignment are separately
correct and according to guidelines.
Poor coordination of horizontal and vertical alignments
can create locations where the available sight distance
drops below the required sight distance (16)

20.  Paved/Unpaved (earth, turf, gravel)
 Graded and usable width depends on
foreslope and rounding
 Consider function, safety, and capacity
impacts
 Slope
 2 to 6% (paved)
 4 to 6% (gravel)
 8% (turf)
 Min. 2% + lane slope
 Max crossover 8%]
 Width 0.6 – 3.6 m (2 – 12 ft)
Shoulder Characteristics
Basic Definitions

21. Shoulder Characteristics
As width is increased, the number of accidents are
decreased
Shoulder Elevation Change may be the result of an
irregularity in the road surface, or where the vehicle’s wheels
drop off the roadway onto a soft shoulder or grass berm.
20 to 25 % of rollover crashes fall into this category. (3)
Larger accident rates are exhibited on un-stabilized
shoulders, including loose gravel, crushed stone, raw
earth or turf, than on stabilized or paved shoulders. (8)
Source of Risk

22. Shoulder Characteristics (Con.)
21% reduction of total accidents was determined
on road with shoulders of 0.9 m-2.7 m compared
to road without shoulders. (15)
One foot widening results in a reduction of
signal-vehicle accidents by 8.8%. (15)
How to improve!

23. Median
Presence of a median on a highway
contributes positively to road safety
Medians designed at a lower altitude than the pavement
are better for safety than medians designed at a higher
altitude.(1)
Median works as division for traffic in opposite
directions and a recovery area for out-of-control
vehicles. (6)

24. Median Characteristics (Con.)
The type of the median barrier is also
an important aspect
The fact that an obstacle is placed within the roadway
environment that provides a target for collisions can lead
to an increased number of crashes. [39]
Different Types (especially concrete) have the potential
to increase crashes. [22]
Since Studies have shown that The presence of a barrier
will result in a reduction of cross-median type crashes but it
also has the potential to increase median-related crashes. (15)
Median Barrier
Safety shape barriers are designed to mitigate the energy
of crash impacts

25. CONCLUSION
 Road improvements are always feasible
 While designing carriageway, Generosity in not always
welcomed.
 Geometric defects and Pavement condition should be
included in accident investigation forms.
 Improvements don’t have to be expensive.

26. references
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and M. Ergun, Technical University of Istanbul, Faculty of Civil Engineering, 34469 Istanbul, Turkey.
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