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Worked Example: Highway Safety Modeling

Worked Example: Highway Safety Modeling

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Worked Example: Highway Safety Modeling

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  1. Worked Example: Highway Safety Modeling

  2. 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

  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. End End T9 T3 C8 T7 C2 C6 T5 T3 C4 T1 C2 T1 Start Start • 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 • CMF = 1.07 • Target = ORR • ORR = 0.177 • CMF Total = 1.012

  19. Step 5: Determine CMFs • Shoulder Widths • Option 1 = 2.5 meters • CMF = 0.86 • Target = ORR • ORR = 0.177 • CMF Total = 0.975

  20. Step 5: Determine CMFs • Horizontal Alignment (for curves only) • 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 • 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 • CMF = 1.22 • Target = ALL

  23. Step 5: Determine CMFs • Roadside Hazard Rating • Option 1 = RHR = 3 • CMF = 1.00 • Target = ALL

  24. Step 5: Determine CMFs • Design Consistency • Base Case • C2 = 1.326 • C4 = 1.642 • C6 = 1.421 • C8 = 1.200 • Option 1 • C2 = 0.990

  25. Step 5: Calculate Composite CMF • 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: FrequencyCollision 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.