1. 2-1 LOW COST SAFETY IMPROVEMENTS The Tools – Identification of High Crash Locations – Session #2

2. 2-2 Identification of High Crash Locations Learning Outcomes: 1.Identify appropriate Engineering Countermeasures by using the 6 Step Crash Mitigation Process

3. 2-3 HSIP Planning Detailed Engineering Study Document the Analysis Project Selection, Implementation, & Evaluation Document the Evaluation Network Screening or Site Selection List of Sites for Review

4. 2-4 Conventional Screening  Conventional techniques of screening use are known to have difficulties in identifying ‘ unsafe ’ sites: ð Crash counts = bias to high volume sites ð Crash rates = bias to low volume sites ð Crash rates ’ assumption of linearity is invalid ð Regression-to-mean (RTM) effect if sufficient allowance is not made for random errors

5. 2-5 What is Network Screening?  Highway network system made up of segments and intersections  Network screening is a systematic examination of all entities  Purpose: To rank all entities, based on selected criteria, in order to conduct detailed safety studies Money should go where it achieves the greatest effect in terms of preventing crashes and reducing their severity http://www.ncdot.org//planning/statewide/gis/DataDist/CountyMap.html

6. 2-6 PSI Index Approach  New and different approach to identify the Potential for Safety Improvement (PSI)  Application of Safety Performance Functions (SPFs) for each crash severity class for different reference groups  PSI values for fatal, injury and PDO crashes combined = PSI Index  Weights (relative cost) are applied to PSI values

7. 2-7 PSI Index Screening  PSI Index is estimated for each location  Locations are ranked in descending order of PSI Index values  Locations with largest PSI Index values have most potential for crash reduction

8. 2-8 SafetyAnalyst Tools  Network screening to identify sites with promise for safety improvement  Diagnosis of safety concerns  Selection of countermeasures  Economic appraisal of countermeasures  Priority ranking of countermeasures  Evaluation of implemented projects

9. 2-9 The six steps in the crash mitigation process 1.Identify Sites 2.Collect Crash Experience 3.Gather Field Conditions 4.Identify Contributing Factors and Countermeasures 5.Assess and Select Countermeasures 6.Implement and Evaluate

10. 2-10 Step 1: Identify Sites with Potential Safety Problems  Crash data  Traffic Measures  Field Observations  Complaints  Enforcement input  Surrogate measures

11. 2-11 Step 1: Identify Sites with Potential Safety Problems Crash Information Methods:  Total Number of Crashes  Crash Density (Crashes per mile)  Crash Rate (Crashes per million vehicle miles)  Number Quality Control  Rate Quality Control  Crash Severity  Severity Index  Crash Index

12. 2-12 Crash Rate Calculation Roadway Segment: CR = N / [ADT x 365 x L x 10 -6 ] where CR is expressed as “Crashes per million vehicle miles (or kilometers)” and N = Number of crashes per year ADT = Average Daily Traffic L = Length of segment (mi or km)

13. 2-13 Crash Rate Calculation: Segment Example: For N = 50 crashes for 3 year period ADT = 3,000 Average Daily Traffic L = 2.8 miles CR = N / [ADT x 365 x L x 10 -6 ] CR = 50 / [3,000 ADT x 3 yrs x 365 x 2.8 miles x 10 -6 ] CR = 5.43 crashes per million vehicle miles

14. 2-14 Average Crash Rates

15. 2-15 Step 1: Identify Sites with Potential Safety Problems *From Mn DOT Traffic Safety Fundamentals Handbook

16. 2-16 Crash Rate Calculation Intersection: CR = N / {[Sum (ADT)s /2] x 365 x 10 -6} where CR is expressed as “crashes per million entering vehicles” and N = Number of crashes per year Sum (ADT)s = Sum of all Average Daily Traffic entering the intersection

17. 2-17 Crash Rate Calculation: Intersection Example: For N = 25 crashes for 3 years ADT (N) = 10,000 ADT (S) = 9,000 ADT (E) = 3,500 ADT (W) = 4,000 Sum (ADT)s=(10,000+9,000+3,500+4,000) = 26,500 CR = N / {[Sum (ADT)s /2] x 365 x 10 -6} CR = 25/ {26,500/2] x 3 yrs x365 x 10 -6} CR = 1.72 crashes per million entering vehicles

18. 2-18 Collision Types at Rural Intersections

19. 2-19 Average Intersection Crash Rates Average of 1.5 crashes per year for Un- Signalized Intersections in rural areas– recent California analysis* Average of 2.5 crashes per year in urban areas *NCHRP 500, Volume 5: A Guide for Addressing Unsignalized Intersection Collisions, 2003.

20. 2-20 Severity Index Severity index (SI) is the ratio of crashes involving an injury or fatality to total crashes

21. 2-21 Step 2: Characterize the Crash Experience Prepare a Collision Diagram

22. 2-22 References- Appendix A: Non-Crash Based Procedures

23. 2-23 Step 3: Characterize Field Conditions

24. 2-24 Step 3: Characterize Field Conditions

25. 2-25 Step 4: Identify Contributing Factors and Appropriate Countermeasures *See Appendix for Tables 12, 13, 14 and 15

26. 2-26 Step 5: Assess Countermeasures and Select Most Appropriate  See Appendix Tables 12, 13, & 14 for “Contributing Factors” Poor Delineation Page 11-14 of Appendicies

27. 2-27 Step 5: Assess Countermeasures and Select Most Appropriate  go to Table of Crash Reduction Factors in the Appendicies, page 11-22, Warning Signs, 3 rd row

28. 2-28 Step 6: Implement Countermeasures and Evaluate Effectiveness Improve Delineation - Advance Warning Sign for Curve with Advisory Speed Plaque CRF = 22%

29. 2-29 Traffic Data Studies: Step 3: Characterize Field Conditions  Traffic Volume – Turning Movement, ADT  Spot Speeds  Traffic Conflict Study  Sight Distance Evaluation

30. 2-30 Traffic Crash Costs: AIS Level Cost Per Injury SeverityDescriptor (Dollars) AIS 3Serious Injury 175,000 AIS 4Severe 565,000 AIS 5Critical2,290,000 AIS 6Fatal3,000,000 Property Damage Only 65,000

31. 2-31 Identification of High Crash Locations – Exercise III  For the Countermeasure Advance Curve Warning Signs + Advisory Speed Plaque = 22%; Calculate the benefit $$ for “before period” of 7 crashes Discussion

32. 2-32 Introduction – Exercise III  For the Countermeasure Advance Curve Warning Signs + Advisory Speed Plaque = 22% Discussion Benefit for “Before Period of 7 Crashes = 7 crashes x CRF of 22% = 7 x 0.22 = 1.54 crashes reduced Severity distribution = 0.32 (Inj+Fatal) + 0.68 Pdo Benefit = (1.54 x 0.32 x $565,000) + (1.54 x 0.68 x $65,000) = $345,100

33. 2-33 Appendix H: Typical Benefit/Cost Ratios Identification of High Crash Locations * From MN DOT Traffic Safety Fundamentals Handbook

34. 2-34 Identification of High Crash Locations Review Questions:  What are the six steps in the crash mitigation process?

35. 2-35 Review: What are the six steps in the crash mitigation process? 1.Identify Sites 2.Collect Crash Experience 3.Gather Field Conditions 4.Identify Contributing Factors and Countermeasures 5.Assess and Select Countermeasures 6.Implement and Evaluate

36. 2-36 Identification of High Crash Locations Review Questions:  What low cost safety measure has the highest benefit to cost ratio? Lighting CRF = 50+%

37. 2-37 Identification of High Crash Locations Learning Outcomes: 1.Identify appropriate Engineering Countermeasures by using the 6 Step Crash Mitigation Process

38. 2-38 Questions? Identification of High Crash Locations