Uses of Safety Performance Functions and Potential for Safety Improvement Values David L. Piper, P.E. Illinois Department of Transportation.

Uses ofSafety Performance

Functionsand

Potential for Safety Improvement Values

David L. Piper, P.E.Illinois Department of

Transportation

Applications of SPF and PSI

• Using the HSIP Five Percent Report• Safety Analysis in Phase I• Quantitative Site Analysis

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Using the HSIP Five Percent Report

• Is my location a “5%” site?– Map– Illinois Roadway Information System

(IRIS) location

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Five Percent Reporting

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PSI = 152 Fixed Object and Overturn = 51% Rear End and Same

Direction Sideswipe = 35%

Five Percent Reporting

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Driving too fast for conditions = 45%

Listing of Crashes

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06-0698

Using SPF and PSI Information in Phase I

• Creation of the 5% Report requires a look at 100% of sites.

• Weighting of PSI supports goal to reduce K’s and A’s

• Substantive safety measure at project level

• Breakdown by segments and intersections within the project

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Using SPF and PSI Information in Phase I

• Suggested triggers for Road Safety Assessment if:– PSI is 10 or higher– Segment or intersection in top 33% of

its peer group– If segment or intersection has PSI 50%

higher than adjacent similar location(s)

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Quantitative Site Analysis

• Is this intersection performing poorly?

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Thru AADT = 2575Stop AADT = 2650Experience = 7 crashes in 5 years. (‘04 – ‘08)1.4 crashes per year1 Fatal CrashRecent PDO crash in 12/08, and A-Injury crash in 01/09

HSM: N spf 4ST = exp[-8.56 + 0.60xln(AADT maj ) + 0.61xln(AADTmin)]= 2.61 crashes/year (No night crashes)

IDOT SPF: N K+I = exp(-8.05)*((AADT major)^0.674)*((AADT minor)^0.272) = 0.55 K+I crashes per 5 years.

Quantitative Site Analysis

• Countermeasures Completed– Lighting– Improve sight distance (hedge clearing)– Relocated utility sign

• Countermeasures Under Consideration– Improved warning signs– Police private signs on ROW

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Quantitative Site Analysis

• Is this intersection performing poorly?

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Thru AADT = 2300Stop AADT = 650Experience = 10 crashes in 5 years. (‘04 – ‘08)2.0 crashes per year1 Fatal Crash3 A-Injury Crashes2B-Injury Crashes

HSM: N spf 4ST = exp[-8.56 + 0.60xln(AADT maj ) + 0.61xln(AADTmin)]= 1.04 crashes/year

IDOT SPF: N K+I = exp(-8.05)*((AADT major)^0.674)*((AADT minor)^0.272) = 0.34 K+I crashes per 5 years.

Quantitative Site Analysis

• Countermeasures Completed– Removed trees in sight triangle

Countermeasures Under Consideration– Improved warning signs– Police private signs on ROW– Lighting

• Overall – Quantitative analysis supports actions taken, and informs future decisions.

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Summary

• SPF/PSI Products Support– Identification of safety opportunities– IDOT goal to reduce K’s and A’s– Office review of 5% locations– Focus of resources to best effect– Credibility of analysis

• SPF/PSI Products will Support– SafetyAnalyst– Highway Safety Manual

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Thank [email protected]

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