Safety Analyst Overview

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• Safety management software for state and local
  highway agencies
• Improves identification and programming of
  site-specific highway safety improvements
• Incorporates state-of-the-art safety management
  approaches with computerized analytical tools
• http//www.safetyanalyst.org/

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Who is DevelopingSafetyAnalyst?

• Federal Highway Administration
• Technical Working Group
• 19 participating (pooled-fund) States
• 1 local highway agency
• 2 MPOs

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Participating States
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SafetyAnalyst Modules

• Module 1 Network Screening
• Module 2 Diagnosis and Countermeasure Selection
• Module 3 Economic Appraisal and Priority
  Ranking
• Module 4 Countermeasure Evaluation

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Status of Development

• Module 1 - Network Screening
• Beta testing of interim version underway
• Module 2 - Diagnosis and Countermeasure Selection
• Interim version available for testing Dec 2006
• Module 3 - Economic Appraisal and Priority
  Ranking
• Interim version available for testing Dec 2006
• Module 4 Countermeasure Evaluation
• Beta testing of interim version underway

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Advantages of SafetyAnalyst over Existing
Techniques

• Integrates/automates all parts of safety
  management process
• Applies state-of-the-art analytical procedures
• Strong cost-effectiveness component
• Enables engineers to make more informed
  decisions more efficiently

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Module 1Network Screening

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Module 1 - Objectives

• Screen entire roadway network, or portion of
  network, and to identify sites with potential for
  safety improvement
• Rank sites with potential for safety improvement
• Select sites for further investigation within
  Module 2 - Diagnosis and Countermeasure Selection

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Types of Network Screening

• Basic network screening
• With Peak Searching on roadway segments
• With Sliding Window on roadway segments
• High proportion of specific accident type
• Sudden increase in mean accident frequency
• Steady increase in mean accident frequency
• Corridors with promise

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Basic Network Screening

• Uses Empirical Bayes methodology
• Combine observed and predicted accidents
• Expected accident values expressed as
• Expected accident frequency
• Excess accident frequency
• Two screening approaches for roadway segments
• Peak searching
• Sliding window

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Basic Network Screening(with Peak Searching on
Roadway Segments)

• For roadway segments, individual sites are
  divided into windows of size 0.1 mi
• Accident frequencies are calculated for each
  window within a site
• Windows are flagged when
• Expected value greater than user-specified limit
• Expected value is statistically reliable
• If no windows are flagged, incrementally increase
  window size by 0.1 mi and test again
• More than one window pertaining to a site can be
  flagged
• Rank order site based upon expected or excess
  accident frequencies

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Peak Searching Concepts
Roadway Segment
Win 1
Win 2
0.03 mi
0.07 mi
Win 3
Win 4
Note Window length 0.1 mi Expected accidents
(acc/mi) Limiting Value 5 acc/mi CVLimit 0.5
Win 5
Win 6
Win 7
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Peak Searching Concepts
Note Window length 0.2 mi Expected accidents
(acc/mi) Limiting Value 5 acc/mi CVLimit 0.5
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Peak Searching Concepts
Note Window length 0.3 mi Expected accidents
(acc/mi) Limiting Value 5 acc/mi CVLimit 0.5
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Peak Searching Concepts
Note Window length 0.4 mi Expected accidents
(acc/mi) Limiting Value 5 acc/mi CVLimit 0.5
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Peak Searching Concepts
Note Window length 0.5 mi Expected accidents
(acc/mi) Limiting Value 5 acc/mi CVLimit 0.5
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Peak Searching Concepts
Note Window length 0.6 mi Expected accidents
(acc/mi) Limiting Value 5 acc/mi CVLimit 0.5
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Peak Searching Concepts
Note Window length Segment length Expected
accidents (acc/mi) Limiting Value 5
acc/mi CVLimit 0.5
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Basic Network Screening(with Sliding Window on
Roadway Segments)

• Users specifies
• Window size (e.g., 0.3 mi)
• Increment length (e.g., 0.1 mi)
• For each window position, determine
• Expected or excess accident frequency
• Windows allowed to bridge sites
• More than one window pertaining to a site can be
  flagged
• Rank order site based upon expected or excess
  accident frequencies

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Sliding Window Concepts
Site No. 1
MP 1.0
MP 2.6
1.1 mi
1.2 mi
1.3 mi
1.4 mi
1.5 mi
First Sliding Window W 0.3 mi
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Sliding Window Concepts
Site No. 1
Second Sliding Window W 0.3 mi
Sliding window is moved incrementally by 0.1 mi
along the roadway segment.
MP 1.0
MP 2.6
1.1 mi
1.2 mi
1.3 mi
1.4 mi
1.5 mi
First Sliding Window W 0.3 mi
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Site No. 23
Site No. 24
MP 35.4
MP 36.2
MP 36.7
Note Window length 0.3 mi Increment length
0.1 mi Expected accidents (acc/mi) Limiting
Value 5 acc/mi
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Site No. 23
Site No. 24
MP 35.4
MP 36.2
MP 36.7
Note Window length 0.3 mi Increment length
0.1 mi Expected accidents (acc/mi) Limiting
Value 5 acc/mi
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Site No. 23
Site No. 24
MP 35.4
MP 36.2
MP 36.7
Note Window length 0.3 mi Increment length
0.1 mi Expected accidents (acc/mi) Limiting
Value 5 acc/mi
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High Proportions of Specific Accident Type

• Objective
• Identify sites having higher than expected
  proportions of specific target accidents
• Rank sites based on difference observed
  proportion and expected proportion of target
  accident
• Methodology
• Calculate observed proportion (TOT only)
• Calculate the probability that observed
  proportion is greater than limiting proportion
  (i.e., avg for site accident type)
• Site flagged when probability is greater than
  some user-specified significance level

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High Proportions of Specific Accident Type (cont.)

• Roadway segments
• Similar to sliding window approach
• Longer windows are needed to reduce variance
  (e.g., 1.0 mi)
• More than one window pertaining to a site can be
  flagged
• Site ranked based upon maximum difference between
  observed proportion and expected proportion

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Sudden Increase in Mean Accident Frequency

• Screening for safety deterioration
• Calculate differences in mean yearly accident
  frequencies
• For the time period with the largest difference
• If the percentage increase is greater than a
  user-specified limiting value
• Then perform test of significance
• Based on observed accidents
• Based on total accidents
• Flagged sites are not rank ordered

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Steady Increase in Mean Accident Frequency

• Screening for safety deterioration
• Fit regression model to data of accident
  frequency versus year
• If value of slope is greater than a
  user-specified limiting slope
• Then perform test of significance
• Based on observed accidents
• Based on total accidents
• Flagged sites are not rank ordered

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Screening for Corridors with Promise

• Analysis of extended corridors (e.g., 10 mi or
  more)
• Roadway segments, intersections, and ramps
  grouped together
• Rank order corridors based upon
• Accidents/mi/yr
• Accidents/million veh-mi/yr
• Based on observed accidents

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Demonstration of Module 1 Network Screening
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SafetyAnalyst Modules

• Module 1 Network Screening
• Module 2 Diagnosis and Countermeasure Selection
• Module 3 Economic Appraisal and Priority
  Ranking
• Module 4 Countermeasure Evaluation

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Module 2 Diagnosis and Countermeasure Selection

• Display collision diagram (links to third-party
  software)
• Identify collision patterns
• Conduct diagnostic investigations
• Suggest countermeasures that address identified
  collision patterns
• Select appropriate countermeasures

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Module 3 Economic Appraisal and Priority Ranking

• Perform economic analysis of alternative
  countermeasures for a specific site
• Perform economic analysis of improvements across
  selected sites
• Select mix of sites and countermeasures to get
  maximum benefits within a specified budget
• Develop priority ranking of alternative
  improvements

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Module 4Countermeasure Evaluation Tool

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Objective

• Determine safety effectiveness (percent reduction
  in crashes) for specific implemented
  countermeasures
• Conduct before-after evaluation of crash
  frequencies using the Empirical Bayes (EB)
  approach
• Conduct before-after evaluation of shifts in
  crash severity or crash type proportion

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Why the Evaluation Tool?

• The goal of SafetyAnalyst is to help highway
  agencies determine how funds can be spent in the
  most cost-effective manner to improve safety.
• The results of Module 4 can be used to update the
  accident modification factors (AMFs) that are
  used within Module 3 for economic appraisal and
  priority ranking of countermeasures to be
  implemented at sites.

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When to Use the Evaluation Tool

• A countermeasure has been implemented at a number
  of sites
• The agency wants to assess how effectively the
  countermeasure performed
• Did it improve the safety performance at a site?
• Did it reduce a specific target accident type?

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What Information Is Needed

• Locations of improved sites
• Countermeasure(s) to evaluate for each site
• Year of implementation
• Countermeasure name(s)
• Site characteristics
• ADTs (before and after improvement)
• Yearly accident counts (before and after)
• Safety Performance Functions (SPFs)

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Types of Analyses Conducted

• The safety effectiveness of countermeasures is
  quantified through the use of before-after
  statistical evaluations.
• Two types of before-after evaluations can be
  conducted
• Percent change in accident frequencies, due to
  the implemented countermeasure, is evaluated by
  an Empirical Bayes (EB) technique.
• Shift in proportion of specific collision types
  is evaluated using the Wilcoxon signed rank test

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Demonstration of Module 4 Countermeasure
Evaluation Tool