worked example: highway safety modeling

Worked Example:Worked Example: Highway Safety Modeling Highway Safety Modeling

OutlineOutline– Safety Modeling Safety Modeling

» Safety Modeling ProcessSafety Modeling Process

– Set-up for Worked ExampleSet-up for Worked Example– Develop / Build Safety ModelDevelop / Build Safety Model

» Project SegmentationProject Segmentation» Selecting and Applying CPMsSelecting and Applying CPMs» Selecting and Applying CMFsSelecting and Applying CMFs

– Estimating Safety PerformanceEstimating Safety Performance– Collision Costs / Economic AnalysisCollision Costs / Economic Analysis– SummarySummary

Safety ModelingSafety Modeling Involves two componentsInvolves two components

– Application of “base” CPMsApplication of “base” CPMs» CPM is used to estimate the expected normal safety CPM is used to estimate the expected normal safety

performance for the facility performance for the facility

– Application of CMFsApplication of CMFs» CMF is combined with the with the CPM to estimate the CMF is combined with the with the CPM to estimate the

safety associated with the specific design features of the safety associated with the specific design features of the facility facility

Methodology follows FHWA - IHDSM and will Methodology follows FHWA - IHDSM and will be the recommended approach in the HSMbe the recommended approach in the HSM

Somewhat analogous to ‘traffic modeling’Somewhat analogous to ‘traffic modeling’

Safety Modeling ProcessSafety Modeling Process

Systematic process for ‘Safety Modeling’Systematic process for ‘Safety Modeling’– Step 1: Understand Project/Limits and SegmentationStep 1: Understand Project/Limits and Segmentation

– Step 2: Assemble requisite data (volume, design,…)Step 2: Assemble requisite data (volume, design,…)

– Step 3: Apply “Base” collision prediction modelStep 3: Apply “Base” collision prediction model

– Step 4: Select design features to include in Safety ModelStep 4: Select design features to include in Safety Model

– Step 5: Calculate the CMFs for selected design featuresStep 5: Calculate the CMFs for selected design features

– Step 6: Estimate safety performanceStep 6: Estimate safety performance

– Step 7: Calculate collision costsStep 7: Calculate collision costs

Set-Up for Worked ExampleSet-Up for Worked Example Consider 2 alignments:Consider 2 alignments:

– 1) Existing Alignment (Base Case)1) Existing Alignment (Base Case)» Characterized by poor horizontal alignment and reduced Characterized by poor horizontal alignment and reduced

cross-sectional dimensionscross-sectional dimensions

– 2) Proposed Improved Alignment (Option 1)2) Proposed Improved Alignment (Option 1)» Characterized by improved horizontal alignment and Characterized by improved horizontal alignment and

increased cross-sectional designincreased cross-sectional design

Objective: Objective: – To estimate the safety benefits associated with the To estimate the safety benefits associated with the

proposed new alignment and the corresponding proposed new alignment and the corresponding collision cost savingscollision cost savings

» Business Case, MAE, Project Justification…Business Case, MAE, Project Justification…

Start

End

T1

C2

T3

Start

End

C2

T1

T3 C4

C6

C8

T5

T7

T9

Existing Road Existing Road “Base Case”“Base Case”

Improved Road Improved Road “Option 1”“Option 1”

Set-Up for Worked ExampleSet-Up for Worked Example Existing HighwayExisting Highway

– RAU2 HighwayRAU2 Highway

– Existing traffic volumeExisting traffic volume» 7500 AADT7500 AADT

– Poor geometryPoor geometry» 5 Tangents / 4 Curves5 Tangents / 4 Curves

» Sub-standard curvesSub-standard curves

» Steep gradesSteep grades

– Cross-sectionCross-section» 3.0 meter lanes3.0 meter lanes

» 1.5 meter shoulders1.5 meter shoulders

» Hazardous roadsideHazardous roadside

Improvement Option 1Improvement Option 1– RAU2 HighwayRAU2 Highway

– Existing traffic volumeExisting traffic volume» 7500 AADT7500 AADT

– Favorable geometryFavorable geometry» 2 Tangents / 1 curve2 Tangents / 1 curve

» Exceed design criteriaExceed design criteria

» Reduced gradesReduced grades

– Cross-sectionCross-section» 3.6 meter lanes3.6 meter lanes

» 2.5 meter shoulders2.5 meter shoulders

» Improved roadsideImproved roadside

Step 1: Project SegmentationStep 1: Project Segmentation

Start and end points for the safety model must be Start and end points for the safety model must be the same for a fair comparisonthe same for a fair comparison

Segmentation of corridor should be primarily Segmentation of corridor should be primarily based on horizontal alignmentbased on horizontal alignment– Tangent 1 / Curve 2 / Tangent 3 / ….Tangent 1 / Curve 2 / Tangent 3 / ….

Segmentation could also be based on significant Segmentation could also be based on significant changes in the design or operationchanges in the design or operation– Change in traffic volumeChange in traffic volume

– Change in design elements (e.g., tunnel)Change in design elements (e.g., tunnel)

– Others as required Others as required

Step 1: Project SegmentationStep 1: Project Segmentation

Step 2: Assemble Required DataStep 2: Assemble Required Data

Step 3: Select and Apply “Base” CPMStep 3: Select and Apply “Base” CPM Select CPM for each option / each facility: Select CPM for each option / each facility:

– ““Existing” and “Proposed Improvement”Existing” and “Proposed Improvement”– Use CPM to calculate the expected normal Use CPM to calculate the expected normal

collision frequency collision frequency – Corridors are:Corridors are:

» Segment / Rural / Arterial / Undivided / 2-Lane Segment / Rural / Arterial / Undivided / 2-Lane

» Use RAU2 models (PDO and Severe)Use RAU2 models (PDO and Severe)

Step 3: Select and Apply “Base” CPMStep 3: Select and Apply “Base” CPM

Step 4: Select Features for AnalysisStep 4: Select Features for Analysis Need to determine what design elements should be Need to determine what design elements should be

included in the safety modelincluded in the safety model Typically include (segments):Typically include (segments):

– Lane widths (Lane widths (✔✔))

– Shoulder widths (Shoulder widths (✔✔))

– Horizontal curve (Horizontal curve (✔✔))

– Grade (Grade (✔✔))

– Access FrequencyAccess Frequency

– Roadside Hazard Level (Roadside Hazard Level (✔✔))

– Median TreatmentMedian Treatment

– Design Consistency (Design Consistency (✔✔) )

Step 5: Determine CMFsStep 5: Determine CMFs Lane WidthLane Width

– Base Case Base Case = 3.0 meters= 3.0 meters

– Option 1Option 1 = 3.6 meters = 3.6 meters


– Base Case Base Case = 3.0 meters= 3.0 meters» CMF CMF TargetTarget = 1.30 = 1.30

» Target = OR + HOTarget = OR + HO

» OR + HO = 0.347OR + HO = 0.347

» CMF CMF TotalTotal = 1.104 = 1.104


– Option 1Option 1 = 3.6 meters = 3.6 meters» CMF CMF TargetTarget = 1.01 = 1.01

» Target = OR + HOTarget = OR + HO

» OR + HO = 0.347OR + HO = 0.347


Step 5: Determine CMFsStep 5: Determine CMFs

Shoulder Widths Shoulder Widths – Base Case Base Case = 1.5 meters= 1.5 meters

– Option 1Option 1 = 2.5 meters = 2.5 meters


Shoulder Widths Shoulder Widths – Base CaseBase Case = 1.5 meters = 1.5 meters

» CMF = 1.07CMF = 1.07

» Target = ORRTarget = ORR

» ORRORR = 0.177= 0.177



Shoulder Widths Shoulder Widths – Option 1 Option 1 = 2.5 meters= 2.5 meters

» CMF = 0.86CMF = 0.86

» Target = ORRTarget = ORR

» ORRORR = 0.177= 0.177



Horizontal Alignment (for curves only)Horizontal Alignment (for curves only)– Base CaseBase Case

» C2 = 1.24C2 = 1.24 Target = ALLTarget = ALL




– Option 1Option 1» C2 = 1.01C2 = 1.01 Target = ALLTarget = ALL

€

CMFHC =0.962Lc +

80.2

3.28R− 0.012S

0.962Lc


Roadway GradeRoadway Grade– Base CaseBase Case

» T1 to T5 = 6% Grade, CMF = 1.100, Target = ALLT1 to T5 = 6% Grade, CMF = 1.100, Target = ALL

» T7 to T9 = 8% Grade, CMF = 1.137, Target = ALLT7 to T9 = 8% Grade, CMF = 1.137, Target = ALL

– Option 1Option 1» T1 and T3 = 2% Grade, CMF = 1.033, Target = ALLT1 and T3 = 2% Grade, CMF = 1.033, Target = ALL


Roadside Hazard RatingRoadside Hazard Rating– Base Case Base Case = RHR = 6= RHR = 6

» CMF = 1.22CMF = 1.22

» Target = ALLTarget = ALL


Roadside Hazard RatingRoadside Hazard Rating– Option 1Option 1 = RHR = 3 = RHR = 3

» CMF = 1.00CMF = 1.00

» Target = ALLTarget = ALL

Step 5: Determine CMFsStep 5: Determine CMFs Design ConsistencyDesign Consistency• Base CaseBase Case

C2 = 1.326 C2 = 1.326 C4 = 1.642 C4 = 1.642 C6 = 1.421C6 = 1.421 C8 = 1.200C8 = 1.200

Option 1Option 1

C2 = 0.990 C2 = 0.990

€

CMF = exp 0.0049×(V85 − Vd )+ 0.0253ΔV85 −1.177ΔfR[ ]

Step 5: Calculate Composite CMFStep 5: Calculate Composite CMF Calculate composite CMFCalculate composite CMF

€

CMFC =CMF1 x CMF2 x..... x CMFn

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

Step 7: Calculate Collision CostsStep 7: Calculate Collision Costs

With safety performance known, it is possible to With safety performance known, it is possible to calculate the collision costs associated with each calculate the collision costs associated with each design scenario.design scenario.

Use BC MOT average collision cost valuesUse BC MOT average collision cost values– Fatal collisionFatal collision = $5,600,000 / incident= $5,600,000 / incident

– Injury collision Injury collision = $100,000 / incident= $100,000 / incident

– P.D.O. collisionP.D.O. collision = $7,350 / incident= $7,350 / incident

Use collision severity distribution to determine the Use collision severity distribution to determine the average cost of a severe collision (F + I)average cost of a severe collision (F + I)– (F + I) collision(F + I) collision = $290,000 / incident = $290,000 / incident

Step 7: Calculate Collision CostsStep 7: Calculate Collision Costs

Base Case:Base Case: FrequencyFrequency Collision CostCollision Cost– PDO Collisions / yrPDO Collisions / yr = 6.0 / yr= 6.0 / yr = $44,000 / yr= $44,000 / yr

– F + I Collisions / yrF + I Collisions / yr = 4.4 / yr= 4.4 / yr = $1,276,000 / yr= $1,276,000 / yr

– Total Collisions / yrTotal Collisions / yr = 10.4 / yr= 10.4 / yr = $1,320,000 / yr= $1,320,000 / yr

Option 1:Option 1:– PDO Collisions / yrPDO Collisions / yr = 2.8 / yr= 2.8 / yr = $21,000 / yr= $21,000 / yr

– F + I Collisions / yrF + I Collisions / yr = 2.1 / yr= 2.1 / yr = $609,000 / yr= $609,000 / yr

– Total Collisions / yrTotal Collisions / yr = 4.9 / yr= 4.9 / yr = $630,000 / yr= $630,000 / yr

Safety Benefit: Option 1Safety Benefit: Option 1 = 5.5 / yr= 5.5 / yr = $690,000 / yr = $690,000 / yr

Step 7: Calculate Collision CostsStep 7: Calculate Collision Costs Also possible to calculate the life-cycle collision Also possible to calculate the life-cycle collision

costs (discounted) as inputs to a MAE, a business costs (discounted) as inputs to a MAE, a business cases or other project justifications.cases or other project justifications.

Safety Model is run for Safety Model is run for – Opening Day;Opening Day;

– Horizon Year; Horizon Year;

– Any interim years when road changes (that affect safety Any interim years when road changes (that affect safety performance) are madeperformance) are made

Collision costs can be reduced to a NPV and Collision costs can be reduced to a NPV and combined with other project evaluation criteriacombined with other project evaluation criteria– Mobility, environmental, economic development, etc. Mobility, environmental, economic development, etc.

SummarySummary CPMs and CMFs can be used to develop a ‘Safety CPMs and CMFs can be used to develop a ‘Safety

Model’ that allows for the explicit quantification Model’ that allows for the explicit quantification of safety performanceof safety performance– CPM estimates the ‘normal’ safety performanceCPM estimates the ‘normal’ safety performance

– CMF estimates how each design feature affects safety CMF estimates how each design feature affects safety

Safety modeling considers the specific design Safety modeling considers the specific design features of a facility to estimate the collision features of a facility to estimate the collision frequency frequency

Results can be converted into collision costs and Results can be converted into collision costs and combined with other evaluation criteria to assess combined with other evaluation criteria to assess and justify highway improvement expenditure. and justify highway improvement expenditure.

worked example: highway safety modeling

Documents

safety modelstep

safety modelingstep

safety modeltypically

safety benefits

safety performancestep

design elements

specific design features

improved alignment option