Network Screening

Module 3 Network Screening 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

Roadway Safety Management Process Source: Highway Safety Manual (HSM), Figure 4-1

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)

Why is Network Screening Important? Maximizes return on safety investments -> maximizes the likely reductions in fatalities and serious injuries. Results allows for • Formulating and implementing policies • Prioritizing systemic improvements

Types of Network Screening Methods • Hot Spot • System wide • Systemic

Hot Spot Analysis Identification 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 Determine Common Contributing Factors Apply System-Wide Proactive 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 system Can be used in policy development

Systemic 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 countermeasures • to be implemented across a system

Network screening STEPS

Network Screening5 key steps

STEP 1Establish Focus Identify the purpose or intended outcome of the network screening analysis. • Overall crash reduction • Policy 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

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

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

STEP 3Select Performance Measures Examples from the HSM (Table 4-2 p. 4-9) • Number of crashes • Crash severity • Critical crash rate • Excess predicted crash frequency using Safety Performance Functions (SPFs)

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)

Greater Reliability More data; account for RTM bias HSM Table 4-2 (p. 4-9)

What are the limitations of using crash rates as a performance measure? Example of a safety performance function (SPF) for a particular facility type and site conditions: notice the non-linear relationship Relationship between crashes and traffic volumes are rarely linear Crashes have variability and randomness

What are our options if traffic volume data are limited? One example of an approach: Develop reasonable estimations of average daily traffic • Allow for relative comparison • Basis for area wide network screening Develop a plan for improving data over time

STEP 4Select Screening Method

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’)

ExampleSignalized Intersection Network

ExampleRanking by crash frequency as a performance measure

FHWA Systemic Tool & usRAP EXAMPLES OF Network screening tools

FHWA Systemic Tool • Systemic Safety Improvement: • An improvement widely implemented based on: • High-risk roadway features • Correlated with high severity particular crash types. Source: http://safety.fhwa.dot.gov/systemic/

FHWA Systemic Tool Source: http://safety.fhwa.dot.gov/systemic/ 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.

FHWA Systemic Tool Source: http://safety.fhwa.dot.gov/systemic/

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

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

Summary Purpose of network screening Basic steps in network screening Safety performance measures & selection criteria Network screening tool examples

End Module 3 Questions?

Network Screening