Role of Safety Performance Functions in the Highway Safety Manual

July 29, 2009

Presentation Overview

• SPF Application in Part C: Predictive Method

• SPF Application in Part B: Roadway Safety Management Process

Safety Performance Functions in

Part C – Predictive Method

SPFs in Part C – Predictive Method

• 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

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)

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

Safety Performance Functions in

Part B – Roadway Safety Management

SPFs and Part B – Roadway Safety Management

Chapter 4 – Network Screening

Chapter 4 – Network Screening

Chapter 4 – Network Screening

• Excess Predicted Average Crash Frequency Using SPFs

• Estimate Crash Reduction on Suburban Arterial

Intersection

Chapter 7 – Economic Appraisal

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

Estimate Monetary Benefit

• Convert crash difference to monetary benefits and estimate annual savings of improvement

Crash SeverityComprehensive

Crash Costs

Fatality (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.

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

• Thank you!

www.highwaysafetymanual.org

SPFs in Part C – Predictive Method

Expected Average Crash Frequency from EB Approach

Predicted Average Crash Frequency from SPF Equation

Observed Crash Counts

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

Chapter 9 - Safety Effectiveness Evaluation