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LOW COST SAFETY IMPROVEMENTS

The Tools – Identification of High Crash

Locations

– Session #2

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Identification of High Crash Locations

Learning Outcomes:

1. Identify appropriate Engineering Countermeasures by using the 6 Step Crash Mitigation Process

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HSIP Planning

Detailed Engineering StudyDocument the Analysis

Project Selection, Implementation, & Evaluation

Document the Evaluation

Network Screening or Site Selection

List of Sites for Review

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Conventional Screening

Conventional techniques of screening use are known to have difficulties in identifying ‘unsafe’ sites: Crash counts = bias to high volume sites Crash rates = bias to low volume sites Crash rates’ assumption of linearity is invalid Regression-to-mean (RTM) effect if sufficient

allowance is not made for random errors

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What is Network Screening? Highway network system

made up of segmentsand intersections

Network screening is a systematic examination of all entities

Purpose: To rank all entities, based on selected criteria, in order to conduct detailed safety studies

Money should go where it achieves the greatest effect in terms Money should go where it achieves the greatest effect in terms of preventing crashes and reducing their severityof preventing crashes and reducing their severity

http://www.ncdot.org//planning/statewide/gis/DataDist/CountyMap.html

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PSI Index Approach

New and different approach to identify the Potential for Safety Improvement (PSI)

Application of Safety Performance Functions (SPFs) for each crash severity class for different reference groups

PSI values for fatal, injury and PDO crashes combined = PSI Index

Weights (relative cost) are applied to PSI values

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PSI Index Screening

PSIIndex is estimated for each location

Locations are ranked in descending order of PSIIndex values

Locations with largest PSIIndex values

have most potential for crash reduction

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SafetyAnalyst Tools

Network screening to identify sites with promise for safety improvement

Diagnosis of safety concernsSelection of countermeasuresEconomic appraisal of countermeasuresPriority ranking of countermeasuresEvaluation of implemented projects

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The six steps in the crash mitigation process

1. Identify Sites2. Collect Crash

Experience3. Gather Field Conditions4. Identify Contributing

Factors and Countermeasures

5. Assess and Select Countermeasures

6. Implement and Evaluate

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Step 1: Identify Sites with Potential Safety Problems

Crash data Traffic

Measures Field

Observations Complaints Enforcement

input Surrogate

measures

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Step 1: Identify Sites with Potential Safety Problems

Crash Information Methods:

Total Number of Crashes Crash Density (Crashes per mile) Crash Rate (Crashes per million vehicle miles) Number Quality Control Rate Quality Control Crash Severity Severity Index Crash Index

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Crash Rate Calculation

Roadway Segment:

CR = N / [ADT x 365 x L x 10-6]

where CR is expressed as “Crashes per million vehicle miles (or kilometers)” and

N = Number of crashes per year

ADT = Average Daily Traffic

L = Length of segment (mi or km)

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Crash Rate Calculation: Segment

Example: For N = 50 crashes for 3 year period ADT = 3,000 Average Daily Traffic L = 2.8 miles

CR = N / [ADT x 365 x L x 10-6]CR = 50 / [3,000 ADT x 3 yrs x 365 x 2.8

miles x 10-6]CR = 5.43 crashes per million vehicle

miles

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Average Crash Rates

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Step 1: Identify Sites with Potential Safety Problems

*From Mn DOT Traffic Safety Fundamentals Handbook

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Crash Rate Calculation

Intersection:

CR = N / {[Sum (ADT)s /2] x 365 x 10-6}

where CR is expressed as “crashes per million entering vehicles” and

N = Number of crashes per year

Sum (ADT)s = Sum of all Average Daily Traffic entering the intersection

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Crash Rate Calculation: Intersection

Example: For N = 25 crashes for 3 years ADT (N) = 10,000 ADT (S) = 9,000 ADT (E) = 3,500 ADT (W) = 4,000Sum (ADT)s=(10,000+9,000+3,500+4,000) = 26,500

CR = N / {[Sum (ADT)s /2] x 365 x 10-6}

CR = 25/ {26,500/2] x 3 yrs x365 x 10-6}

CR = 1.72 crashes per million entering vehicles

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Collision Types at Rural Intersections

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Average Intersection Crash Rates

• Average of 1.5 crashes per year for Un-Signalized Intersections in rural areas– recent California analysis*

• Average of 2.5 crashes per year in urban areas

*NCHRP 500, Volume 5: A Guide for Addressing Unsignalized Intersection Collisions, 2003.

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Severity Index

Severity index (SI) is the ratio of crashes involving an injury or fatality to total crashes

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Step 2: Characterize the Crash Experience

Prepare a Collision Diagram

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References- Appendix A:

Non-Crash Based Procedures

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Step 3: Characterize Field Conditions

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Step 3: Characterize Field Conditions

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Step 4: Identify Contributing Factors and Appropriate Countermeasures

*See Appendix for Tables 12, 13, 14 and 15

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Step 5: Assess Countermeasures and Select Most Appropriate

See Appendix Tables 12, 13, & 14 for “Contributing Factors”

Poor Delineation

Page 11-14 of Appendicies

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Step 5: Assess Countermeasures and Select Most Appropriate

go to Table of Crash Reduction Factors in the Appendicies, page 11-22, Warning Signs, 3rd row

Warning Signs and Measures

Advance Warning Signs (General) 25%

Advance Curve Warning Signswith Advisory Speed Plaques 22%

Advance Curve Warning Signswithout Advisory Speed Plaques 18%

Chevron Alignment Warning Signs 33% - 49%

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Step 6: Implement Countermeasures and Evaluate Effectiveness

Improve Delineation - Advance Warning Sign for Curve with Advisory Speed Plaque CRF = 22%

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Traffic Data Studies:

Step 3: Characterize Field Conditions

Traffic Volume – Turning Movement, ADT

Spot Speeds

Traffic Conflict Study

Sight Distance Evaluation

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Traffic Crash Costs:

AIS Level Cost Per InjurySeverity Descriptor (Dollars)

AIS 3 Serious Injury 175,000

AIS 4 Severe 565,000

AIS 5 Critical 2,290,000

AIS 6 Fatal 3,000,000

Property Damage Only 65,000

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Identification of High Crash Locations – Exercise III

For the Countermeasure Advance Curve Warning Signs + Advisory Speed Plaque = 22%; Calculate the benefit $$ for “before period” of 7 crashes

Discussion

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Introduction – Exercise III

For the Countermeasure Advance Curve Warning Signs + Advisory Speed Plaque = 22%

Discussion

Benefit for “Before Period of 7 Crashes

= 7 crashes x CRF of 22%

= 7 x 0.22 = 1.54 crashes reduced

Severity distribution = 0.32 (Inj+Fatal) + 0.68 Pdo

Benefit = (1.54 x 0.32 x $565,000) + (1.54 x 0.68 x $65,000) = $345,100

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Appendix H: Typical Benefit/Cost Ratios

Identification of High Crash Locations

* From MN DOT Traffic Safety Fundamentals Handbook

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Identification of High Crash Locations

Review Questions:

What are the six steps in the crash mitigation process?

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Review: What are the six steps in the crash mitigation process?

1. Identify Sites2. Collect Crash

Experience3. Gather Field Conditions4. Identify Contributing

Factors and Countermeasures

5. Assess and Select Countermeasures

6. Implement and Evaluate

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Identification of High Crash Locations

Review Questions:

What low cost safety measure has the highest benefit to cost ratio?

Lighting CRF = 50+%

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Identification of High Crash Locations

Learning Outcomes:

1. Identify appropriate Engineering Countermeasures by using the 6 Step Crash Mitigation Process

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Questions?

Identification of High Crash Locations