1. Network Screening 1 Module 3 Safety Analysis in a Data-limited, Local Agency Environment: July 22, 2013 - Boise, Idaho

2. 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 2

3. Roadway Safety Management Process Network Screening ___________Chapter 4 Diagnosis _________Chapter 5 Select Countermeasures __________Chapter 6 Economic Appraisal ________Chapter 7 Prioritize Projects ________Chapter 8 Safety Effectiveness Evaluation ____________Chapter 9 Source: Highway Safety Manual (HSM), Figure 4-1 3

4. 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) 4

5. 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 5

6. Types of Network Screening Methods Hot Spot System wide Systemic

7. 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

8. System-Wide 5-8 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

9. Systemic 5-9 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

10. NETWORK SCREENING STEPS 10

11. 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 Screening 5 key steps 11

12. STEP 1 Establish Focus Identify the purpose or intended outcome of the network screening analysis.  Overall crash reduction  Policy implementation 12

13. Example Focus: 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 13

14. 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 14

15. 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 15

16. STEP 3 Select 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) 16

17. STEP 3 Select 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) 17

18. HSM Table 4-2 (p. 4-9) More data; account for RTM bias Greater Reliability 18

19. 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 19 Example of a safety performance function (SPF) for a particular facility type and site conditions: notice the non-linear relationship

20. 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 20

21. STEP 4 Select Screening Method Sliding window Peak searching Segments Simple ranking Intersections Sliding window Peak searching Simple ranking Corridors 21

22. STEP 5 Screen / 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’) 22

23. Example Signalized 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 23

24. Example Ranking by crash frequency as a performance measure IntersectionTotal Crashes 1257 2322 3119 480 554 6315 729 8447 9214 1050 24

25. EXAMPLES OF NETWORK SCREENING TOOLS FHWA Systemic Tool & usRAP 25

26. FHWA Systemic Tool Systemic Safety Improvement:  An improvement widely implemented based on: High-risk roadway features Correlated with high severity particular crash types. 26 Source: http://safety.fhwa.dot.gov/systemic/

27. 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/ 27

28. FHWA Systemic Tool Source: http://safety.fhwa.dot.gov/systemic/ 28

29. 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 29

30. 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 30

31. Summary Purpose of network screening Basic steps in network screening Safety performance measures & selection criteria Network screening tool examples 31

32. End Module 3 Questions? 32