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Role of Safety Performance Functions in the Highway Safety Manual
July 29, 2009
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Presentation Overview
• SPF Application in Part C: Predictive Method
• SPF Application in Part B: Roadway Safety Management Process
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Safety Performance Functions in Part C – Predictive Method
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SPFs in Part C – Predictive Method
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• SPF for specific facility type
• AADT
• Length
• Site characteristics to adjust with AMFs– Roadway– Intersection
• Local Calibration Factor
• Crash data for EB Method application
Summary Data Needs for SPF Application
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Application of SPFs in Part C 1.5-mi rural two-lane Tangent roadway segment 10,000 veh/day 0% grade Local calibration factor =
1.10
6 driveways per mi 10-ft lane width 4-ft gravel shoulder Roadside hazard rating = 4 Nobserved = 10 crashes/year
• Objective:- Calculate predicted average crash frequency (Npredicted)
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Step 1 - Base Conditions Estimate (Nspf)
Predictive Method Sample Application
4
Design RoadsideDensity DrivewayWidth/Type ShoulderWidth LaneCOMB AMF AMFAMF AMF AMF
1.10 4 1.38 Npredicted
312.0e 6spfrs 10365LAADTN
(-0.312)-6 e 103651.510,000 Crashes/year
1.38
Crashes/year6
Step 2 – Estimate Predicted Average Crash Frequency
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Safety Performance Functions in
Part B – Roadway Safety Management
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SPFs and Part B – Roadway Safety Management
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Chapter 4 – Network Screening
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Chapter 4 – Network Screening
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Chapter 4 – Network Screening
• Excess Predicted Average Crash Frequency Using SPFs
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• Estimate Crash Reduction on Suburban Arterial Intersection
Chapter 7 – Economic Appraisal
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Estimate Crash Reduction• Predicted Crashes - INTERSECTION
– Predict Future Crash Frequency with SPF
Data needs: SPFs, Future AADT, Length
– Apply AMFs and Calibration Factor to Account for Local Conditions
Data needs: AMFs for existing condition, local calibration factor
• Predicted Crashes - ROUNDABOUT
– Apply AMF to Account for Alternate Conditions (Roundabout)
Data needs: AMFs for alternate condition
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Estimate Monetary Benefit• Convert crash difference to monetary benefits
and estimate annual savings of improvement
Crash SeverityComprehensive
Crash CostsFatality (K) $4,008,900
Disabling Injury (A) $216,000
Evident Injury (B) $79,000
Fatal/Injury (K/A/B) $158,200
Possible Injury (C) $44,900
PDO (O) $7,400Source: Crash Cost Estimates by Maximum Police-Reported Injury Severity within Selected Crash Geometries, FHWA - HRT - 05-051, October 2005.
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Summary of Data Needs for SPF Application
• Applicable SPF
• AADT
• Length
• Local Geometric/Operational Conditions and Corresponding AMFs
• Local Calibration Factors
• Historical crash data for EB Method Application
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SPFs in Part C – Predictive Method
Expected Average Crash Frequency from EB Approach
Predicted Average Crash Frequency from SPF Equation
Observed Crash Counts
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observedpredictedexpected N x w) - (1 N x wN
100.507-16.0840.507Nexpected
8 Crashes/year
Predictive Method Sample Application
• Step 3 – Estimate Expected Average Crash Frequency
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Chapter 9 - Safety Effectiveness Evaluation