Bloody RoutesBy

Mona Khafagy, MSc., P.E.

Highway and Transportation Expert

Research and Teaching Associate

American University in Cairo

2016

Effect of Defects in Road Geometric

Elements on Accident RatesContents

Carriageway

Horizontal Curve

Vertical Curve

Combined Horizontal and Vertical Curves

Shoulders and Medians

Conclusion

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.

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

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

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??

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 RiskHow Defects in Cross Slope Affect Accident !!!!

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!

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

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!

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

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!

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

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.)

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

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

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!

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)

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)

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

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

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!

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)

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

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.

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Civil and Environmental Engineering, University College Cork, Ireland,2010 .3. “The Effect of Cross-Sectional Geometry on Heavy Vehicle Performance and Safety”, Paul Milliken and John de Pont, TERNZ Ltd,

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