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NC Local Safety Partnership Selecting Interventions.

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Presentation on theme: "NC Local Safety Partnership Selecting Interventions."— Presentation transcript:

1 NC Local Safety Partnership Selecting Interventions

2 Workshop Roadmap Program Background and Overview Understanding Crash Data Identifying Potential Treatment Locations Preparing Collision Diagrams Selecting Interventions Evaluation Methods Program Implementation and Discussion 2

3 Module Objectives ■ Use safety data and site investigation to explore problem areas ■ Identify contributing crash factors and select appropriate countermeasures ■ Conduct benefit-cost analysis to guide countermeasure implementation 3

4 Steps for Selecting Interventions Review safety dataInvestigate site Identify potential contributing factors Identify and list potential countermeasures Conduct benefit-cost analysis 4

5 STEP 1 – REVIEW SAFETY DATA Module 5 – Selecting Interventions 5

6 Safety Data Review ■ Descriptive statistics ■ Crash type ■ Crash severity (KABCO) ■ Environmental conditions ■ Event sequence ■ Crash locations ■ Supporting documentation 6

7 Descriptive Statistics: Crash Type 7

8 Descriptive Statistics: Crash Severity 8

9 Descriptive Statistics: Environmental Conditions 9

10 Event Sequence 10

11 Crash Locations: Collision Diagram 11

12 Supporting Documentation ■ Land use maps ■ Historic weather patterns ■ Public comment records ■ Roadway improvement plans ■ Anecdotal information 12

13 STEP 2 – INVESTIGATE SITE Module 5 – Selecting Interventions 13

14 Site Investigation ■ Investigate site to observe field conditions ■ Verify if site characteristics agree with details on crash report ■ Drive both directions of a road segment or each approach of an intersection ■ Observe traffic movements and signal timing at various points during the day ■ Evaluate sight distance 14

15 Site Investigation ■ Note locations of driveways and other access points ■ Observe use by non-motorized users (i.e., where are pedestrians crossing?) ■ Take photos ■ Look for evidence of safety issues (tire marks on curb, pole damage, broken glass on pavement, etc.) ■ Construct condition diagram for later reference 15

16 Condition Diagram Light Pole Pothole Driveway Tree Sidewalk School Playground Bank 16

17 Example of Verified Field Review ■ Signalized intersection ■ Left turn lanes in all quadrants ■ Permissive/protected phasing ■ Pedestrian crossings and signal control ■ Clearance interval not adequate for prevailing speeds 17

18 STEP 3 – IDENTIFY CONTRIBUTING FACTORS Module 5 – Selecting Interventions 18

19 Identify Contributing Factors Roadway HumanVehicle 19

20 ■ Potential contributing factors are provided in the AASHTO Highway Safety Manual for: ■ Roadway Segments ■ Signalized Intersections ■ Unsignalized Intersections ■ Highway-Rail Grade Crossings ■ Pedestrians ■ Bicyclists Contributing Crash Factors 20

21 Example Possible Contributing Factors Signalized Intersections (See p. 6-5 and 6-6 of the HSM ) 21

22 Contributing Factors for Severe Crash Types Angle and turning (left) ■ Misjudging speed of opposing traffic ■ Visibility of signals ■ Inadequate signal timing Rear end (east approach) ■ Inappropriate or excessive approach speeds ■ Unexpected stops (driveways/access) 22

23 Example ■ Two-way stop-controlled intersection ■ Four legs ■ Major road AADT = 22,100 ■ Minor road AADT = 1,650 ■ Urban area 23

24 Example 3-Year Crash Total Predominant Crash Types Crash Severity FatalInjuryPDO 35Angle, Head- on 6%65%29% What are the likely contributing crash factors? 24

25 STEP 4 – IDENTIFY AND LIST POTENTIAL COUNTERMEASURES Module 5 – Selecting Interventions 25

26 Sources for Potential Countermeasures ■ Roadway Countermeasures ■ NCHRP Series 500 ■ Highway Safety Manual, Part D ■ FHWA Crash Modification Factor Clearinghouse ■ FHWA list of suggested (proven) countermeasures ■ Behavioral Countermeasures ■ Countermeasures That Work 26

27 Countermeasure Sources: NCHRP Series 500 Guides Countermeasures classified as: 1.Proven, 2.Tried, or 3.Experimental Examples: Relocate roadside objects (P) Install shoulder rumble strips (T) Delineate poles with retroreflective tape (E) 27

28 Countermeasure Sources: Highway Safety Manual ■ First edition released in 2010 ■ Provides practitioners with the best factual information and tools regarding safety consequences of design decisions. ■ Sections ■ Part A: Safety knowledge ■ Part B: Safety management ■ Part C: Crash prediction models ■ Part D: Countermeasure selection and CMFs 28

29 Countermeasure Sources: Crash Modification Factors Clearinghouse 29

30 Countermeasure Sources: FHWA Suggested Countermeasures (2008) ■ Road safety audits ■ Roundabouts ■ Left and right turn lanes at stop-controlled intersections ■ Yellow and all red change intervals at traffic signals ■ Median and pedestrian refuge areas in urban and suburban areas ■ Walkways 30

31 Countermeasure Sources: NHTSA “Countermeasures That Work” 31

32 Question ■ When faced with many potential countermeasures, how do you choose which one(s) to implement? 32

33 Comparing Countermeasures ■ Subjective comparisons are important… ■ Which will garner the most public support? ■ Which is most appropriate for the area? ■ …But objective comparisons are priority ■ Expected effectiveness -> decreases in crashes (CMFs) ■ Expected costs -> installation and maintenance 33

34 STEP 5 – CONDUCT BENEFIT- COST ANALYSIS Module 5 – Selecting Interventions 34

35 Countermeasure Costs ■ Startup or installation costs ■ Example? ■ Ongoing operational or maintenance costs ■ Example? ■ Resilience/staying power (“usable life”) ■ Which countermeasures would have shorter staying power? Which would be longer? 35

36 Countermeasure Benefits ■ Crashes prevented – use CMFs to estimate if available ■ Changes in crash severity ■ Ex: Signals and red light cameras ■ Ex: Cable median barriers ■ Other benefits not related to safety (e.g., reduced delay) 36

37 Countermeasure Benefits: Crash Modification Factors ■ Crash modification factor (CMF) is a multiplicative factor used to compute the expected number of crashes after implementing a given countermeasure at a specific site. ■ CMF = ■ CMF > 1 indicates an expected increase in crashes ■ CMF < 1 indicates an expected decrease in crashes Expected crashes with countermeasure Expected crashes without countermeasure 37

38 Countermeasure Benefits: Crash Modification Factors 38

39 Countermeasure Benefits: Assigning Monetary Value to Crashes Prevented ■ Complex Process ■ Rules of Thumb ■ Fatal plus Serious Injury vs. minor injury plus PDO Costs ■ Cost Effectiveness 39

40 Countermeasure Benefits: NCDOT Crash Cost Resource 40

41 Countermeasure Benefits: NCDOT Crash Costs 2011 41

42 Benefit-Cost Analysis ■ Striving for the most effective use of limited safety funds (“bang for the buck”) ■ Rank competing projects ■ Methods ■ Benefit-cost ratio ■ Present value of benefits 42

43 Example: Benefit-Cost Ratio ■ Proposed: Install left turn lane at signalized intersection ■ Cost: ■ $168,000 for construction and $32,000 for right-of-way ■ Annual cost over 22-year service life is $19,700 ■ Benefit: ■ Expected to reduce 6 crashes per year ■ Equivalent to $187,300 in annual crash cost savings 43

44 Example: Benefit-Cost Ratio ■ Annual cost = $19,700 ■ Annual benefit = $187,300 ■ Benefit-cost ratio 44 187,300 19,700 = 9.5

45 Other (Qualitative) Considerations ■ What are other considerations that can play a role in which countermeasures are implemented? 45

46 Other (Qualitative) Considerations ■ Design Standards ■ Tradeoffs ■ Familiarity ■ Constituent Concerns 46

47 Take Away Messages ■ Reviewing safety data (and supporting documentation) is a critical first step ■ Field reviews reveal problems that may not be apparent in data review ■ By identifying other contributing factors, countermeasures can be selected to best address crash problem ■ Benefit-cost analysis can help prioritize and guide improvements 47


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