Worked Example: Highway Safety Modeling

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

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’

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

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…

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”

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 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

Step 1: Project Segmentation

Step 2: Assemble Required Data

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)

Step 3: Select and Apply “Base” CPM

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 (✔)

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

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

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

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

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

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

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

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

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

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

Step 5: Determine CMFs Design Consistency Base Case Option 1

Step 5: Calculate Composite CMF

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

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

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

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

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.

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.