1. Worked Example: Highway Safety Modeling

2. Outline Safety Modeling Set-up for Worked Example
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

3. 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
Methodology follows FHWA - IHDSM and will be the recommended approach in the HSM
Somewhat analogous to ‘traffic modeling’

4. Safety Modeling Process
Systematic process for ‘Safety Modeling’
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

5. 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…

6. Existing Road “Base Case”
Start
End C2 T1 T3 C4 C6 C8 T5 T7 T9
Start
End T1 C2 T3
Existing Road “Base Case”
Improved Road “Option 1”

7. 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

8. Step 1: Project Segmentation
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 / ….
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

9. Step 1: Project Segmentation

10. Step 2: Assemble Required Data

11. Step 3: Select and Apply “Base” CPM
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)

12. Step 3: Select and Apply “Base” CPM

13. Step 4: Select Features for Analysis
Need to determine what design elements should be included in the safety model
Typically include (segments):
Lane widths (✔)
Shoulder widths (✔)
Horizontal curve (✔)
Grade (✔)
Access Frequency
Roadside Hazard Level (✔)
Median Treatment
Design Consistency (✔)

14. Step 5: Determine CMFs Lane Width Base Case = 3.0 meters
Option 1 = 3.6 meters

15. Step 5: Determine CMFs Lane Width Base Case = 3.0 meters
CMF Target = 1.30
Target = OR + HO
OR + HO = 0.347
CMF Total = 1.104

16. Step 5: Determine CMFs Lane Width Option 1 = 3.6 meters
CMF Target = 1.01
Target = OR + HO
OR + HO = 0.347
CMF Total = 1.003

17. Step 5: Determine CMFs Shoulder Widths Base Case = 1.5 meters
Option 1 = 2.5 meters

18. Step 5: Determine CMFs Shoulder Widths Base Case = 1.5 meters
Target = ORR
ORR = 0.177
CMF Total = 1.012

19. Step 5: Determine CMFs Shoulder Widths Option 1 = 2.5 meters
Target = ORR
ORR = 0.177
CMF Total = 0.975

20. Step 5: Determine CMFs Horizontal Alignment (for curves only) Option 1
Base Case
C2 = 1.24 Target = ALL
C4 = 1.88 Target = ALL
C6 = 1.30 Target = ALL
C8 = 1.08 Target = ALL
Option 1
C2 = 1.01 Target = ALL

21. Step 5: Determine CMFs Roadway Grade Option 1 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

22. Step 5: Determine CMFs Roadside Hazard Rating Base Case = RHR = 6
Target = ALL

23. Step 5: Determine CMFs Roadside Hazard Rating Option 1 = RHR = 3
Target = ALL

24. Step 5: Determine CMFs Design Consistency Base Case Option 1

25. Step 5: Calculate Composite CMF

26. Step 6: Estimate Safety Performance
Safety Performance = CPM x CMFs

27. Step 6: Estimate Safety Performance
Safety Performance = CPM x CMFs

28. Step 7: Calculate Collision Costs
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
Use collision severity distribution to determine the average cost of a severe collision (F + I)
(F + I) collision = $290,000 / incident

29. Step 7: Calculate Collision Costs
Base Case: Frequency Collision Cost
PDO Collisions / yr = 6.0 / yr = $44,000 / yr
F + I Collisions / yr = 4.4 / yr = $1,276,000 / yr
Total Collisions / yr = 10.4 / yr = $1,320,000 / yr
Option 1:
PDO Collisions / yr = 2.8 / yr = $21,000 / yr
F + I Collisions / yr = 2.1 / yr = $609,000 / yr
Total Collisions / yr = 4.9 / yr = $630,000 / yr
Safety Benefit: Option 1 = 5.5 / yr = $690,000 / yr

30. Step 7: Calculate Collision Costs
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
Collision costs can be reduced to a NPV and combined with other project evaluation criteria
Mobility, environmental, economic development, etc.

31. Summary
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
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.