Worked Example:
Highway Safety Modeling
Outline
– Safety Modeling
» Safety Modeling Process
– Set-up for Worked Example
– Develop / Build Safety Model
» Project Segmentation
» Selecting and Applying CPMs
» Selecting and Applying CMFs
– Estimating Safety Performance
– Collision Costs / Economic Analysis
– Summary
Safety Modeling

Involves two components
– Application of “base” CPMs
» CPM is used to estimate the expected normal safety
performance for the facility
– Application of CMFs
» CMF is combined with the with the CPM to estimate the
safety associated with the specific design features of the
facility
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Methodology follows FHWA - IHDSM and will
be the recommended approach in the HSM
Somewhat analogous to ‘traffic modeling’
Safety Modeling Process
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Systematic process for ‘Safety Modeling’
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Step 1: Understand Project/Limits and Segmentation
Step 2: Assemble requisite data (volume, design,…)
Step 3: Apply “Base” collision prediction model
Step 4: Select design features to include in Safety Model
Step 5: Calculate the CMFs for selected design features
Step 6: Estimate safety performance
Step 7: Calculate collision costs
Set-Up for Worked Example

Consider 2 alignments:
– 1) Existing Alignment (Base Case)
» Characterized by poor horizontal alignment and reduced
cross-sectional dimensions
– 2) Proposed Improved Alignment (Option 1)
» Characterized by improved horizontal alignment and
increased cross-sectional design

Objective:
– To estimate the safety benefits associated with the
proposed new alignment and the corresponding
collision cost savings
» Business Case, MAE, Project Justification…
End
End
T9
T3

Existing Road
“Base Case”
C8
C2
C6
T5
T3
C4
T1
C2
T1
Start
Improved
Road “Option
1”
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T7
Start
Set-Up for Worked Example

Existing Highway
– RAU2 Highway
– Existing traffic volume
» 7500 AADT
– Poor geometry
» 5 Tangents / 4 Curves
» Sub-standard curves
» Steep grades
– Cross-section
» 3.0 meter lanes
» 1.5 meter shoulders
» Hazardous roadside

Improvement Option 1
– RAU2 Highway
– Existing traffic volume
» 7500 AADT
– Favorable geometry
» 2 Tangents / 1 curve
» Exceed design criteria
» Reduced grades
– Cross-section
» 3.6 meter lanes
» 2.5 meter shoulders
» Improved roadside
Step 1: Project Segmentation
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Start and end points for the safety model must be
the same for a fair comparison
Segmentation of corridor should be primarily
based on horizontal alignment
– Tangent 1 / Curve 2 / Tangent 3 / ….
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Segmentation could also be based on significant
changes in the design or operation
– Change in traffic volume
– Change in design elements (e.g., tunnel)
– Others as required
Step 1: Project Segmentation
Step 2: Assemble Required Data
Step 3: Select and Apply “Base” CPM
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Select CPM for each option / each facility:
– “Existing” and “Proposed Improvement”
– Use CPM to calculate the expected normal
collision frequency
– Corridors are:
» Segment / Rural / Arterial / Undivided / 2-Lane
» Use RAU2 models (PDO and Severe)
Step 3: Select and Apply “Base” CPM
Step 4: Select Features for Analysis
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Need to determine what design elements should be
included in the safety model
Typically include (segments):
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Lane widths (✔)
Shoulder widths (✔)
Horizontal curve (✔)
Grade (✔)
Access Frequency
Roadside Hazard Level (✔)
Median Treatment
Design Consistency (✔)
Step 5: Determine CMFs
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Lane Width
– Base Case = 3.0 meters
– Option 1 = 3.6 meters
Step 5: Determine CMFs
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Lane Width
– Base Case = 3.0 meters
»
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CMF Target = 1.30
Target = OR + HO
OR + HO = 0.347
CMF Total = 1.104
Step 5: Determine CMFs
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Lane Width
– Option 1 = 3.6 meters
»
»
»
»
CMF Target = 1.01
Target = OR + HO
OR + HO = 0.347
CMF Total = 1.003
Step 5: Determine CMFs
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Shoulder Widths
– Base Case = 1.5 meters
– Option 1 = 2.5 meters
Step 5: Determine CMFs
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Shoulder Widths
– Base Case = 1.5 meters
»
»
»
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CMF = 1.07
Target = ORR
ORR = 0.177
CMF Total = 1.012
Step 5: Determine CMFs
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Shoulder Widths
– Option 1 = 2.5 meters
»
»
»
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CMF = 0.86
Target = ORR
ORR = 0.177
CMF Total = 0.975
Step 5: Determine CMFs
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Horizontal Alignment (for curves only)
– Base Case
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»
»
»
C2 = 1.24
C4 = 1.88
C6 = 1.30
C8 = 1.08
Target = ALL
Target = ALL
Target = ALL
Target = ALL
– Option 1
» C2 = 1.01
CMFHC 
Target = ALL

80.2
 0.012S
3.28R
0.962Lc
0.962Lc 
Step 5: Determine CMFs
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Roadway Grade
– Base Case
» T1 to T5 = 6% Grade, CMF = 1.100, Target = ALL
» T7 to T9 = 8% Grade, CMF = 1.137, Target = ALL
– Option 1
» T1 and T3 = 2% Grade, CMF = 1.033, Target = ALL
Step 5: Determine CMFs
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Roadside Hazard Rating
– Base Case = RHR = 6
» CMF = 1.22
» Target = ALL
Step 5: Determine CMFs
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Roadside Hazard Rating
– Option 1 = RHR = 3
» CMF = 1.00
» Target = ALL
Step 5: Determine CMFs
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Design Consistency
• Base Case

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
C2 = 1.326
C4 = 1.642 
C6 = 1.421
C8 = 1.200
 Option 1
 C2 = 0.990
CMF  exp0.0049(V85  Vd ) 0.0253V85 1.177fR 
Step 5: Calculate Composite CMF
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Calculate composite CMF

CMFC CMF1 x CMF2 x..... x CMFn
Step 6: Estimate Safety Performance
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Safety Performance = CPM x CMFs
Step 6: Estimate Safety Performance
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Safety Performance = CPM x CMFs
Step 7: Calculate Collision Costs
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With safety performance known, it is possible to
calculate the collision costs associated with each
design scenario.
Use BC MOT average collision cost values
– Fatal collision = $5,600,000 / incident
– Injury collision = $100,000 / incident
– P.D.O. collision = $7,350 / incident
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Use collision severity distribution to determine the
average cost of a severe collision (F + I)
– (F + I) collision = $290,000 / incident
Step 7: Calculate Collision Costs
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Base Case:
– PDO Collisions / yr
– F + I Collisions / yr
– Total Collisions / yr
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Frequency Collision Cost
= 6.0 / yr
= 4.4 / yr
= 10.4 / yr
= $44,000 / yr
= $1,276,000 / yr
= $1,320,000 / yr
– PDO Collisions / yr
– F + I Collisions / yr
– Total Collisions / yr
= 2.8 / yr
= 2.1 / yr
= 4.9 / yr
= $21,000 / yr
= $609,000 / yr
= $630,000 / yr
Safety Benefit: Option 1
= 5.5 / yr
= $690,000 / yr
Option 1:
Step 7: Calculate Collision Costs
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Also possible to calculate the life-cycle collision
costs (discounted) as inputs to a MAE, a business
cases or other project justifications.
Safety Model is run for
– Opening Day;
– Horizon Year;
– Any interim years when road changes (that affect safety
performance) are made
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Collision costs can be reduced to a NPV and
combined with other project evaluation criteria
– Mobility, environmental, economic development, etc.
Summary
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CPMs and CMFs can be used to develop a ‘Safety
Model’ that allows for the explicit quantification
of safety performance
– CPM estimates the ‘normal’ safety performance
– CMF estimates how each design feature affects safety
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Safety modeling considers the specific design
features of a facility to estimate the collision
frequency
Results can be converted into collision costs and
combined with other evaluation criteria to assess
and justify highway improvement expenditure.