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NC Local Safety Partnership. Selecting Interventions. Workshop Roadmap. . . . . Module Objectives. Use safety data and site investigation to explore problem areas Identify contributing crash factors and select appropriate countermeasures
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NC Local Safety Partnership Selecting Interventions
Workshop Roadmap
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
Module 5 – Selecting Interventions STEP 1 – REVIEW SAFETY DATA
Safety Data Review • Descriptive statistics • Crash type • Crash severity (KABCO) • Environmental conditions • Event sequence • Crash locations • Supporting documentation
Supporting Documentation Land use maps Historic weather patterns Public comment records Roadway improvement plans Anecdotal information
Module 5 – Selecting Interventions STEP 2 – Investigate Site
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
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
Condition Diagram Bank School Sidewalk Playground Light Pole Driveway Pothole Tree Sidewalk
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
Module 5 – Selecting Interventions STEP 3 – IDENTIFY CONTRIBUTING FACTORS
Contributing Crash Factors • Potential contributing factors are provided in the AASHTO Highway Safety Manual for: • Roadway Segments • Signalized Intersections • Unsignalized Intersections • Highway-Rail Grade Crossings • Pedestrians • Bicyclists
Example Possible Contributing FactorsSignalized Intersections (See p. 6-5 and 6-6 of the HSM )
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)
Example Two-way stop-controlled intersection Four legs Major road AADT = 22,100 Minor road AADT = 1,650 Urban area
Example What are the likely contributing crash factors?
Module 5 – Selecting Interventions STEP 4 – identify and list potential countermeasures
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
Countermeasure Sources:NCHRP Series 500 Guides • Countermeasures classified as: • Proven, • Tried, or • Experimental • Examples: • Relocate roadside objects (P) • Install shoulder rumble strips (T) • Delineate poles with retroreflective tape (E)
Countermeasure Sources:Highway Safety Manual • First edition released in 2010 • Provides practitioners with the best factual information and tools regarding safety consequencesof design decisions. • Sections • Part A: Safety knowledge • Part B: Safety management • Part C: Crash prediction models • Part D: Countermeasure selection and CMFs
Countermeasure Sources:Crash Modification Factors Clearinghouse
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
Question When faced with many potential countermeasures, how do you choose which one(s) to implement?
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
Module 5 – Selecting Interventions STEP 5 – conduct benefit-cost analysis
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?
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)
Countermeasure Benefits:Crash Modification Factors Expected crashes with countermeasure Expected crashes without countermeasure 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
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
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
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
Example: Benefit-Cost Ratio 187,300 = 9.5 19,700 Annual cost = $19,700 Annual benefit = $187,300 Benefit-cost ratio
Other (Qualitative) Considerations What are other considerations that can play a role in which countermeasures are implemented?
Other (Qualitative) Considerations Design Standards Tradeoffs Familiarity Constituent Concerns
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
