Guide for Quantitative Approaches to Systemic Safety Analysis (2020) / Chapter Skim
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20 Overview of Systemic Safety Management Approaches The general purpose and methodology for performing a systemic safety management analysis are explained in part 2.3 in Section 2, but agencies use a variety of implementation approaches for their systemic safety management programs. Differences in crash data availability, roadway characteristic inventory availability, and scope of the systemic safety program may affect how an agency applies a systemic safety management approach.
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Overview of Systemic Safety Management Approaches 21 This section of the guide primarily focuses on the first element of the Systemic Tool methodology -- the systemic safety planning process. This process contains four basic steps: • Identify target crash types and crash contributing factors.
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22 Guide for Quantitative Approaches to Systemic Safety Analysis will vary from agency to agency, but several of the common crash data elements of interest across agencies include: • Crash severity, • Manner of collision/collision impact, • First harmful event, • Speed-related, • Alcohol involvement, • Drug involvement, • Light conditions, and • Sequence of events. Another option for an agency to identify target crash types to address using a systemic safety management approach is to refer to a state or regional SHSP.
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Overview of Systemic Safety Management Approaches 23 levels combined, fatal and all-injury crashes, and/or fatal and serious-injury crashes)
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24 Guide for Quantitative Approaches to Systemic Safety Analysis • Annual average daily traffic, • Motor vehicle posted/statutory speed limit, • Width of lane(s) and shoulder(s)
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Overview of Systemic Safety Management Approaches 25 Facility Type Crash Type R un -o ffro ad La ne de pa rt ur e H ea don A ng le R ol lo ve r Roadway Segments Rural, two-lane roads on horizontal curve sections X X X X Rural, two-lane roads on tangent sections X X X X X Intersections Four-leg minor-road stop-controlled intersections on rural two-lane roads X Four-leg minor-road stop-controlled intersections on urban two-lane roads X Three-leg minor-road stop-controlled intersections on rural two-lane roads X Four-leg signalized intersections on urban, multilane, divided roads X Four-leg signalized intersections on urban, multilane, undivided roads X Four-leg minor-road stop-controlled intersections on rural, multilane, divided roads X Table 1. Common target crash type and facility type combinations for systemic safety management (Porter et al., forthcoming)
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26 Guide for Quantitative Approaches to Systemic Safety Analysis Target Crash Types Potential Contributing Factors Pedestrians (Roadways) Lane width Shoulder width/type Presence of sidewalk Driveway presence, design, and density Presence of transit stops Presence of lighting Posted speed limit or operating speed Bicyclists (Intersections)
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Overview of Systemic Safety Management Approaches 27 Contributing Factor MIRE Data Element MMUCC Data Element Roadway Segments Number of lanes Number of through lanes (FDE) Total Lanes in Roadway Lane width Outside through-lane widthInside through-lane width Width of Lane(s)
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28 Guide for Quantitative Approaches to Systemic Safety Analysis Posted speed limit or operating speed Speed limit Nighttime speed limit 85th percentile speed Mean speed Motor vehicle posted/statutory speed limit Presence and type of bicycle facility Presence/type of bicycle facility Presence/type of bicycle facility Average daily traffic volumes Annual Average Daily Traffic (AADT)
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Overview of Systemic Safety Management Approaches 29 Minimum Uniform Crash Criteria (MMUCC) related to each contributing factor.
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30 Guide for Quantitative Approaches to Systemic Safety Analysis 3.1.4 Project Prioritization To determine which candidate locations receive different treatments, a decision-making framework should be established. It should be based on the countermeasures selected as well as contributing factors and other characteristics of each candidate location that impact the applicability, effectiveness, and viability of each treatment.
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Overview of Systemic Safety Management Approaches 31 Network screening SPFs can be used to calculate multiple performance measures such as predicted, expected, or excess crash frequencies. One or more performance measures are then selected to rank sites for their potential to reduce future crashes.
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32 Guide for Quantitative Approaches to Systemic Safety Analysis SPFs and EB procedures in most analytical procedures, and includes a systemic site selection module that provides the capability to select an individual countermeasure and identify the most cost-effective sites for implementation. The remainder of this section highlights key features of the Safety Analyst software and the systemic site selection module.
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Overview of Systemic Safety Management Approaches 33 • A test of proportions that measures if the observed proportion of a given collision type at a given site is above the average proportion of the given collision type for similar sites, • Steady or sudden increases in mean crash frequencies, and • Crash frequencies and rates for extended corridors. In Module 2, as part of the diagnostic process, Safety Analyst provides several options for identifying crash patterns of interest for further diagnosis, including: • Crash summary reports, • Collision diagrams, • Safety performance reports, and • Statistical tests.
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34 Guide for Quantitative Approaches to Systemic Safety Analysis • Benefit-cost ratio (expressed as ratio of the present safety benefit of a countermeasure to its construction costs) , • Net benefit (expressed as the difference between safety benefits in monetary terms and construction costs)
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Overview of Systemic Safety Management Approaches 35 countermeasure (e.g., shoulder rumble strips or centerline rumble strips) can be selected for potential implementation, and through minimal input from the user, Safety Analyst can identify the most cost-effective sites to implement the countermeasure.
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36 Guide for Quantitative Approaches to Systemic Safety Analysis of that particular crash type or types. If the CMF for the countermeasure is not for a specific crash type, the network screening analysis ranks the sites with the highest expected or excess crash frequency for total crashes (i.e., all crash types combined)
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Overview of Systemic Safety Management Approaches 37 Data Element Description Roadway Segment Data Segment ID (MIRE FDE) This item is a unique, agency-specific identifier for the roadway segment.
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38 Guide for Quantitative Approaches to Systemic Safety Analysis Data Element Description Roadway Segment Directional Attributes Direction (MIRE FDE) This item identifies one of two opposing directions on the roadway segment to which the subelements and items contained in the directional element apply.
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Overview of Systemic Safety Management Approaches 39 Data Element Description Ramp Data Ramp configuration (MIRE FDE) Describes the characterization of the design of the ramp.
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40 Guide for Quantitative Approaches to Systemic Safety Analysis Data Element Description Crash Data (continued) Relationship to junction Identifies the type of related cross street to the crash site.
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Overview of Systemic Safety Management Approaches 41 Agencies can still use Safety Analyst if they do not have all of the required data elements. For example, if an agency has a roadway inventory database, but not an intersection or ramp inventory database, the agency can still bring its roadway data into the software and use Safety Analyst to analyze roadway segments but not intersections and ramps.
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42 Guide for Quantitative Approaches to Systemic Safety Analysis 3.2.3 Results Provided by Safety Analyst The primary output reports provided by Safety Analyst for the systemic site selection module (Module 5) are described in this section.
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ID Site Type Site Subtype County Route Site Start Location Site End Location Average Observed Crashes for Entire Site1 Location with Highest Potential for Safety Improvement Rank Additional Windows of Interest Average Observed Crashes1 Predicted Crash Frequency1 Expected Crash Frequency Excess Crash Frequency Modified LOSS Start Location End LocationExpected Frequency1 Variance2 No. of Fatalities No.
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Proposed Site-CM Site ID Site Type County Route Beginning Location Ending Location Countermeasure CM Start Location CM End Location Construction Cost for Single Implementation Safety Benefit Present Value of Construction Cost for Analysis Period Net Benefits per Site Net Benefits per Mile Total Crashes Reduced per Site* Total Crashes Reduced per Mile*
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Overview of Systemic Safety Management Approaches 45 priority order for implementation of continuous milled-in shoulder rumble strips based on the total number of crashes reduced per site. Site 7445 represents the highest ranked site for implementation of continuous milled-in shoulder rumble strips.
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46 Guide for Quantitative Approaches to Systemic Safety Analysis • Limited information on reliable countermeasures is incorporated into the software due to the current state of knowledge on countermeasures. • The software requires purchasing an annual license.
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Overview of Systemic Safety Management Approaches 47 3.3.1 Capabilities of usRAP ViDA The ViDA software can be used to perform two types of analysis: • Develop star ratings and • Develop safer roads investment plans. Star ratings and safer roads investment plans are developed for 328-ft (i.e., 100-m)
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48 Guide for Quantitative Approaches to Systemic Safety Analysis – The countermeasure is compatible with other cost-effective countermeasures for the same location. – The countermeasure is not overridden by a mutually exclusive countermeasure for the same location that is more cost effective.
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Overview of Systemic Safety Management Approaches 49 • Roadway type (divided/undivided) • Upgrade cost (extent of roadside development that would influence the cost of installing countermeasures)
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50 Guide for Quantitative Approaches to Systemic Safety Analysis • Maps that can be displayed on the computer screen that show all roads for the road network being analyzed, color coded by star ratings. By default, the maps cover the entire road network, but the software can also create maps for individual routes and individual road sections.
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Overview of Systemic Safety Management Approaches 51 • Input data consists mostly of familiar design and traffic control parameters that can be coded from aerial and street-level photographic images, such as Google Earth or Bing Streetside and can be coded by trained technicians or students with an average of approximately 30 minutes of labor per mile of roadway. • Input data can be readily managed with commercially available spreadsheet software such as Microsoft Excel.
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52 Guide for Quantitative Approaches to Systemic Safety Analysis • The results provide a program of cost-effective potential infrastructure improvements to reduce fatal and serious-injury crashes, with specific countermeasures recommended for implementation at specific locations. • The software includes a large amount of support information and transparency of the process.
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Overview of Systemic Safety Management Approaches 53 Application of FHWA's Systemic Tool Methodology Application of SPFs Application of usRAP Methodology Can be used for Identification of target crash type(s) , facility type(s)
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