safety management software for state and local highway agencies: –improves identification and...

• 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/

http://www.safetyanalyst.org/

Who is DevelopingSafetyAnalyst?

• Federal Highway Administration

• Technical Working Group

•19 participating (pooled-fund) States

•1 local highway agency

•2 MPOs

Participating States

SafetyAnalyst Modules

Module 1 – Network Screening

Module 2 – Diagnosis and Countermeasure Selection

Module 3 – Economic Appraisal and Priority Ranking

Module 4 – Countermeasure Evaluation

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

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

Module 1:Network Screening

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

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

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

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

0.0 mi 0.1 mi 0.2 mi 0.3 mi 0.4 mi 0.5 mi 0.6 mi 0.67 mi

Win # 1

Win # 2

Win # 3

Win # 4

Win # 5

Win # 7

0.03 mi

0.07 mi

Roadway Segment

Win # 6

Note:Window length = 0.1 miExpected accidents = (acc/mi)Limiting Value = 5 acc/miCVLimit = 0.5

Peak Searching Concepts


Win # 1

Win # 2

Win # 3

Win # 4

Win # 5

0.03 mi

Roadway Segment

Win # 6

0.07 mi




Win # 1

Win # 2

Win # 3

Win # 4

Win # 5

Roadway Segment





Win # 1

Win # 2

Win # 3

Win # 4

Roadway Segment




Win # 1

Win # 2

Win # 3

Roadway Segment




Win # 1

Win # 2

Roadway Segment




Win # 1

Roadway Segment

Note:Window length = Segment lengthExpected accidents = (acc/mi)Limiting Value = 5 acc/miCVLimit = 0.5

Site No. 1

MP 1.0 MP 2.6

First Sliding WindowW = 0.3 mi

1.1 mi 1.2 mi 1.3 mi 1.4 mi 1.5 mi

Sliding Window Concepts

Site No. 1

MP 1.0 MP 2.6

First Sliding WindowW = 0.3 mi

1.1 mi 1.2 mi 1.3 mi 1.4 mi 1.5 mi

Second Sliding WindowW = 0.3 mi

Sliding window is moved incrementallyby 0.1 mi along the roadway segment.

Sliding Window Concepts

Site No. 23 Site No. 24

MP 35.4 MP 36.2 MP 36.7

X = 3.1

Note:Window length = 0.3 miIncrement length = 0.1 miExpected accidents = (acc/mi)Limiting Value = 5 acc/mi

X = 4.3

X = 5.6

X = 7.8

X = 9.8

X = 7.5

X = 6.0

X = 4.5

X = 3.5

X = 3.0

X = 4.0

X = 5.1

X = 5.5

Site No. 23 Site No. 24

MP 35.4 MP 36.2 MP 36.7

Note:Window length = 0.3 miIncrement length = 0.1 miExpected accidents = (acc/mi)Limiting Value = 5 acc/mi

X = 5.6

X = 7.8

X = 9.8

X = 7.5

X = 6.0

X = 5.1

X = 5.5

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

• 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

High Proportions of Specific Accident Type (cont.)

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

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

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

Demonstration of Module 1: Network Screening

SafetyAnalyst Modules

Module 1 – Network Screening



Module 4 – Countermeasure Evaluation


• Display collision diagram (links to third-party software)

• Identify collision patterns

• Conduct diagnostic investigations

• Suggest countermeasures that address identified collision patterns

• Select appropriate countermeasures


• 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

Module 4:Countermeasure Evaluation Tool

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

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.

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?

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)

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

Demonstration of Module 4: Countermeasure Evaluation Tool

safety management software for state and local highway agencies: –improves identification and...

Documents

window length

peak searching conceptsnote

network screeningmodule

window size

accmilimiting value

portion of network

miexpected accidents

window pertaining