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2-1
LOW COST SAFETY IMPROVEMENTS
The Tools – Identification of High Crash
Locations
– Session #2
2-2
Identification of High Crash Locations
Learning Outcomes:
1. Identify appropriate Engineering Countermeasures by using the 6 Step Crash Mitigation Process
2-3
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
2-5
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
2-30
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
2-36
Identification of High Crash Locations
Review Questions:
What low cost safety measure has the highest benefit to cost ratio?
Lighting CRF = 50+%
2-37
Identification of High Crash Locations
Learning Outcomes:
1. Identify appropriate Engineering Countermeasures by using the 6 Step Crash Mitigation Process
2-38
Questions?
Identification of High Crash Locations