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Network Screening

Module 3

Safety Analysis in a Data-limited, Local Agency Environment:

July 22, 2013 - Boise, Idaho

Learning Objectives Learn approaches for network screening, process

and when to use them Understand key concepts of network screening

Identifying a focus area Identifying sites with higher potential for safety improvement Safety performance measures Exposure and the value of exposure data in network screening

Network Screening Tools FHWA Systemic tool usRAP Tools software

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Roadway Safety Management Process Network Screening

___________Chapter 4

Diagnosis_________Chapter 5

SelectCountermeasures

__________Chapter 6

Economic Appraisal

________Chapter 7

Prioritize Projects

________Chapter 8

Safety Effectiveness Evaluation

____________Chapter 9

Source: Highway Safety Manual (HSM), Figure 4-13

What is Network Screening?

Review of safety performance of roadway network to identify sites with potential for safety improvement

The product of network screening: a list of sites that could be studied further to identify appropriate countermeasures (site diagnosis)

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Why is Network Screening Important? Maximizes return on safety investments ->

maximizes the likely reductions in fatalities and serious injuries.

Results allows forFormulating and implementing policiesPrioritizing systemic improvements

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Types of Network Screening Methods

Hot Spot System wide Systemic

Hot Spot AnalysisIdentification of hot spot locations based on observed crashes or knowledge of the area Reactive approach

Tend to be high volume or exposure

Few locations have lots of severe crashes, but severe crashes may be distributed throughout

System-Wide

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Determine Common Contributing FactorsApply System-WideProactive approach

Contributing factors based on crash type- Cross-median crashes, cable median barrier- Roadway departure crashes, rumble strips

Method allows for low cost safety countermeasures to be implemented across a systemCan be used in policy development

Systemic

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Determine Common Features that Contribute to Severe Crashes Proactive approach

Based on high-risk roadway features that are correlated with high severity particular crash types

- Curves of a certain radius, visual traps- Intersections along divided roadways, skew angle

Method allows for low cost safety countermeasuresto be implemented across a system

NETWORK SCREENING STEPS

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• Establish Focus

STEP 1.

• Identify Reference Populations

STEP 2. • Select Performance Measures

STEP 3.

• Select Screening Methods

STEP 4. • Screen and Evaluate Results

STEP 5.

Network Screening5 key steps

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STEP 1Establish Focus

Identify the purpose or intended outcome of the network screening analysis. Overall crash reductionPolicy implementation

ExampleFocus: Rumble Strip Applications

Which sites can be improved with rumble strips?

What is the target crash type? (run-off-the-road crashes)

Identify sites with high proportion of run-off-the-road crashes -> apply rumble strips

Photo: WSDOT

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Example Focus for Studies

Two-Lane shoulder widening policy Cable median barrier implementation Road departure (e.g., rumble strips) High risk rural roads Enforcement at segments with records of

high severity or fatal crashes

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STEP 2 Identify Reference Population Identify network elements to be screened

• Examples: • Intersections• Roadway segments• Corridors

Organize by reference population• Examples:

• Four-leg signalized intersections• Curved roadway segments• Two-Lane rural roadway corridors

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STEP 3Select Performance Measures

Examples from the HSM (Table 4-2 p. 4-9)

Number of crashesCrash severityCritical crash rateExcess predicted crash frequency using

Safety Performance Functions (SPFs)

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STEP 3Select Performance Measures

Variety of performance measures, depends on data availability Crash Roadway Exposure data

Most reliable performance measures are those that account for regression-to-the-mean (RTM)

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HSM Table 4-2 (p. 4-9)

Mor

e da

ta;

acco

unt

for

RT

M b

ias

Gre

ater

Rel

iabi

lity

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What are the limitations of using crash rates as a performance measure? Relationship between crashes and traffic

volumes are rarely linear Crashes have variability and randomness

0

5

10

15

20

25

30

0 5000 10000 15000 20000 25000 30000

Cra

sh F

requency

AADT

Example of a safety performance function (SPF) for a particular facility type and site conditions: notice the non-linear relationship

What are our options if traffic volume data are limited?One example of an approach:

1. Develop reasonable estimations of average daily traffic• Allow for relative comparison • Basis for area wide network screening

2. Develop a plan for improving data over time

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STEP 4Select Screening Method

• Sliding window• Peak searchingSegments

• Simple ranking

Intersections

• Sliding window• Peak searching• Simple ranking

Corridors

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STEP 5Screen / Evaluate Results

Calculate performance measure for each segment or node

Rank order locations Proceed to diagnosis and countermeasure

evaluation for highest ranked sites (‘sites with promise’)

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ExampleSignalized Intersection Network

• Signalized Intersections

• Urban and Suburban

• State Routes

Reference population

• Crash Rate• EPDO• EPDO Expected

Crash Frequency with EB Adjustment

Performance Measures: • Simple Ranking

Screening Method

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ExampleRanking by crash frequency as a performance measure

Intersection Total Crashes

1 257

2 322

3 119

4 80

5 54

6 315

7 29

8 447

9 214

10 50

Intersection Ranking

Total Crashes

8 447

2 322

6 315

1 257

9 214

3 119

4 80

5 54

10 50

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EXAMPLES OF NETWORK SCREENING TOOLS

FHWA Systemic Tool & usRAP

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FHWA Systemic Tool

Systemic Safety Improvement: An improvement

widely implemented based on:

High-risk roadway features

Correlated with high severity particular crash types.

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Source: http://safety.fhwa.dot.gov/systemic/

FHWA Systemic Tool

Provides a step-by-step process for conducting systemic safety analysis;

Analytical techniques for determining a balanced implementation;

Quantification of safety benefits of systemic improvements.

Source: http://safety.fhwa.dot.gov/systemic/

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FHWA Systemic Tool

Source: http://safety.fhwa.dot.gov/systemic/

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usRAP Tools Software for Network Screening usRAP Tools software can be applied to review an

entire highway network and identify improvement locations

Software focuses on identifying cost-effective highway infrastructure improvements

Software is simple and easy to use -- anyone can learn to use the software

Required input data can be assembled with about 20 minutes of effort per mile of roadway from Google Street View or other sources

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usRAP Tools Software for Network Screening Software output identifies potential locations for

safety improvement projects, as well as candidate project types

usRAP Tools software is web-based and easily accessible

usRAP Tools software will be demonstrated in Module 6A of this workshop

Software access and training are available through AAA Foundation for Traffic Safety

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Summary Purpose of network screening Basic steps in network screening Safety performance measures & selection

criteria Network screening tool examples

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End Module 3

Questions?

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