Session #3 Network Screening

1. HSM Roadway Safety Management Process Session #3 Network Screening

2. HSM Part B – Network Screening • Describe the five major steps for the Network Screening Process • Establish Focus • Indentify Network & Establish Reference Population • Select Performance Measures • Select Screening Method • Screen and Evaluate Results Learning Outcomes:

3. HSM Part B – Network Screening Six Chapters on the Roadway Safety Management Process • Chapter 4 • Chapter 5 • Chapter 6 • Chapter 7 • Chapter 8 • Chapter 9

4. HSM Part B – Network Screening Network Screening Chapter 4 • Review transportation network • Identify problem locations, road segments, corridors, etc. • Rank the sites based on potential for reducing crash frequency

5. HSM Part B – Network Screening Network Screening Process

6. HSM Part B – Network Screening Step 1. Establish Focus – Intended outcome of the network screening analysis • Overall crash reduction • Policy implementation • Install rumble strips at locations with high ROR crashes

7. HSM Part B – Network Screening Example Safety Policies and Programs • Automated enforcement • Cable median barrier implementation • Public information programs (safety belts, alcohol, young drivers, etc.) • Road departure (e.g. rumble strips) • High risk rural roads

8. HSM Part B – Network Screening Step 2. Identify Reference Population • Identify network elements to be screened • Intersections • Roadway segments • Facilities • Ramps • Ramp terminal intersections • Organize by reference population

9. HSM Part B – Network Screening Identify Reference Population What is the reference population for implementing signalization treatments (e.g., red-light cameras)?

10. HSM Part B – Network Screening Step 3. Select Performance Measures (13) • Average Crash Frequency • Crash Rate • Equivalent Property Damage Only (EPDO) Avg Crash Frequency • Relative Severity Index • Critical Rate • Excess Predicted Average Crash Frequency – Method of Moments • Level of Service of Safety (LOSS) • Excess Predicted Crash Frequency using SPFs • Probability of Specific Crash Types Exceeding Threshold Proportion • Excess Proportion of Specific Crash Types • Expected Average Crash Frequency with EB Adjustment • EPDO Average Crash Frequency with EB Adjustment • Excess Expected Average Crash Frequency with EB Adjustment

11. Table 4-2: Stability of Performance Measures Greater Reliability More data; account for RTM bias

12. Selecting Performance Measures Key Criteria Data availability (see Table 4-1) Crashes Traffic volumes Calibrated Safety Performance functions Severity weighting factors (e.g, crash costs) Regression-to-the-mean bias Accounts for natural variability of crashes Performance threshold Reference point for comparing similar sites HSM Part B – Network Screening

13. Data Needs for Performance Measures Greater Reliability More data; account for RTM bias

14. HSM Part B – Network Screening Two Commonly Used Performance Measures: • Average Crash Frequency • Sites are ranked based on the total number of crashes or by a particular crash severity or type during a given time period.  • Crash Rate • The crash rate normalizes the crash frequency based on exposure (traffic volume)

15. HSM Part B – Network Screening Performance Measure Example: Crash Rate Method • Crash Rate Average Crash Frequency in a Period • Exposure in the Same Period • Average Crash Frequency • Sites with highest Crash Frequency in a given time period = = • Detailed Information on each Performance Measure is provided in the HSM

16. HSM Part B – Network Screening Strengths and Weaknesses

17. HSM Part B – Network Screening Example Use of Crash Rates Before After Gambling Introduced in 1992 Example Provided by Jake Konov, P.E., Ph.D., Colorado DOT

18. HSM Part B – Network Screening Crash Rate Conclusion? Before Gambling Average Rate = 2.28 Highway Alignment and Typical Cross-Section have not Changed After Gambling Average Rate = 1.24 but the Percent of Alcohol Related Crashes increased by 500% Possible Conclusion: Is Drinking and Driving in Concert with Gambling Good for Safety? Probably Not – but crash rates say otherwise Example Provided by Jake Konov, P.E., Ph.D., Colorado DOT

19. HSM Part B – Network Screening If crash data and traffic volume is available – able to use Critical Crash Rate Method • Reduces exaggerated effect of sites with low traffic volumes • Considers variance in crash data • Establishes a threshold for site comparisons • Can be applied to a specific crash type or severity • Does not account for RTM bias

20. HSM Part B – Network Screening Critical Crash Rate • Observed crash rates are compared to a calculated critical crash rate • Sites that exceed their respective critical rate are flagged for further review • Critical crash rate depends on the average crash rate at similar sites • A statistical constant for a desired confidence level (85 to 99.5%) is used to calculate the critical crash rate

21. HSM Part B – Network Screening Step 4. Select Screening Method

22. Screening Method Purpose Identifies locations that is most likely to benefit from a safety countermeasure to reduce: Crash Frequency Crash Severity HSM Part B – Network Screening

23. Performance Measure Consistency with Available Screening Methods

24. Sliding Window Method: A window of specified length is moved along the roadway segment Selected Performance Measure is applied to each position of the window Window that shows the most potential for reduction in crash frequency or severity is identified and used to represent crash reduction for the whole segment Segments with the greatest potential for crash reduction, or severity, are studied in detail to identify potential countermeasures HSM Part B – Network Screening

25. Example: Sliding Window Method There are four 0.30 sub-segments (window positions) on Segment A. Sub-segment 4 from 0.30 mi to 0.60 mi has a potential for reducing the average crash frequency by 1.90 crashes. This sub-segment would be used to define the total segment crash frequency because this is the highest potential for reduction in crash frequency or severity of all four windows. Segment A would be ranked and compared to other segments HSM Part B – Network Screening

26. Peak Searching Method: Segment is subdivided into windows of similar length Selected Performance Measure is applied to each position of the window Window length increases incrementally until the calculated Coefficient of Variation (CV) is equal to or lower than the CV limiting value (default value= 0.5) Large CV = low level precision Small CV = high level precision Window that shows the maximum value for the performance measure is identified and used to represent crash reduction for the whole segment HSM Part B – Network Screening

27. Simple Ranking Method: Simple method that can be applied to nodes (intersections or ramp terminals) Selected Performance Measure is applied to all sites Results are ordered from high to low For segments, results are not as reliable as other screening methods HSM Part B – Network Screening

28. HSM Part B – Network Screening Step 5. Screen / Evaluate Results • Calculate performance measures for each segment or node • Apply selected screening method • Rank order

29. “Screening” of Potential Crash Frequency Improvement Locations • Simple Example (elemental level): • Use the HSM to identify those locations with the largest difference of the Actual Crash Frequency to the Predicted Crash Frequency (simple approach)

30. “Screening” of Potential Crash Frequency Improvement Locations - How do we “choose” or “prioritize” among three (3) intersections…….

31. “Screening” of Potential Crash Frequency Improvement Locations - Ranking by “# of crashes per year”

32. “Screening” of Potential Crash Frequency Improvement Locations - Ranking by “Rate per Million Entering Veh”

33. “Screening” of Potential Crash Frequency Improvement Locations - Ranking by “Predicted Crash Frequency

34. “Screening” of Potential Crash Frequency Improvement Locations - Ranking by “Difference from the Predicted Avg Crash Frequency”

35. HSM Part B – Network Screening • Described the five major steps for the Network Screening Process • Establish Focus • Indentify Network & Establish Reference Population • Select Performance Measures • Select Screening Method • Screen and Evaluate Results Learning Outcomes:

36. HSM Part B – Network Screening Questions and Discussion