Guide for Quantitative Approaches to Systemic Safety Analysis (2020) / Chapter Skim
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From page 61... ...
61 Best Practices This section highlights the experiences and best practices of several state and local highway agencies in implementing systemic safety programs to illustrate the application of a systemic safety approach in three main topic areas: systemic safety implementation approaches, application of systemic safety by local agencies, and evaluation of systemic safety programs. In part 5.1, case studies of agencies using each of the three primary implementation approaches -- the FHWA Systemic Tool, the SPF approach, and the usRAP approach -- are given.
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62 Guide for Quantitative Approaches to Systemic Safety Analysis elements of the systemic safety planning process of the Systemic Tool, highlighting the ways some state and local highway agencies have implemented them. This section focuses on the first few steps in the systemic safety management process as illustrated in Element 1 of Figure 2.
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Best Practices 63 of how the risk ratio was calculated for comparing horizontal curvature sharper than 2 degrees (2,800-ft radius) with horizontal curvature flatter than 2 degrees.
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64 Guide for Quantitative Approaches to Systemic Safety Analysis MnCMAT – Minnesota Crash Mapping Analysis Tool ATP - Area Transportation Partnership Int - Intersection crashes Inters-Related - Intersection-Related crashes Figure 6. Minnesota crash tree example (Scurry and Preston, 2013)
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From page 65... ...
Best Practices 65 lighting with a star. The number of stars associated with each location is used to rank locations based on the presence of identified crash contributing factors.
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66 Guide for Quantitative Approaches to Systemic Safety Analysis to each attribute, which are then summed for each location. As a result, each location has a weighted score based on its unique combination of attributes, with all locations being ranked based on these computed scores.
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Best Practices 67 5.1.1.3 Countermeasure Selection In Washington State, the first systemic treatment project deployed was cable median barrier along freeway segments in 1995. This medium-cost strategy is used to reduce severe roadway departure crashes, being especially effective in addressing cross-median head-on collisions.
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68 Guide for Quantitative Approaches to Systemic Safety Analysis the most appropriate treatment for sites selected. Several countermeasures that agencies have implemented as part of their systemic safety management projects are listed in Table 19.
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Best Practices 69 Figure 9. Minnesota intersection project selection decision tree (MnDOT, 2019)
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70 Guide for Quantitative Approaches to Systemic Safety Analysis $27 million (66%) of its annual HSIP funds to reduce roadway departure crashes.
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Best Practices 71 5.1.2.2 Prioritizing Installation of Select Countermeasures Using SPFs In recent years, the Illinois Department of Transportation (IDOT) has made an effort to broadly deploy rumble strips on rural two-lane roadways across the state.
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72 Guide for Quantitative Approaches to Systemic Safety Analysis of the locations where rumble strips were subsequently installed during the years following the assessment is shown in Figure 11. 5.1.3 Implementing Systemic Safety Using usRAP In 2013, the Utah DOT chose to implement the usRAP program, because it was presented to them as a less resource-intense alternative to Safety Analyst.
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Best Practices 73 5.1.3.1 Identifying and Prioritizing Safety Projects Utah has not implemented an independent systemic safety program. Instead, it uses information gathered from usRAP as one way to justify safety projects.
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74 Guide for Quantitative Approaches to Systemic Safety Analysis Regions generally identify potential projects based on crash history, but when regions need help identifying potential safety projects, the DOT central office provides assistance by suggesting projects recommended by usRAP data. The types of projects suggested often include rumble strips, shoulder widening, guardrail, and intersection turn lanes.
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Best Practices 75 The Utah DOT noted that for projects recommended by usRAP but not supported by either of the other two project justification methodologies (CMFs or HSM approaches) , a minimum benefit-cost ratio of 6.0 is used.
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76 Guide for Quantitative Approaches to Systemic Safety Analysis or in the use of a required data element within a software tool. Priorities can also be established to develop a more comprehensive dataset -- first, for a portion of the network, then for other parts of the network.
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Best Practices 77 5.2.1 Pedestrian Corridors Pedestrian crashes tend to be widespread in dense urban environments where a variety of pedestrian generators, such as office buildings, shopping centers, and schools, create high volumes of pedestrian traffic. However, due to the complex nature of pedestrian crashes and their associated contributing factors, pedestrian crashes are often difficult to predict based exclusively on crash history.
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Figure 13. Minnesota urban intersections: pedestrian/bike-related safety project decision tree (MnDOT, 2019)
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Best Practices 79 Figure 14. Distribution of Alamo Area Metropolitan Planning Organization severe (KA)
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80 Guide for Quantitative Approaches to Systemic Safety Analysis Similarly, MnDOT utilized a systemic safety management approach to assess intersections on its local rural system as a part of its 2019 County Roadway Safety Plan. Based on historic crash data, a selection of crash contributing factors were identified to assess which intersections exhibited the greatest potential for crashes (Table 22)
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From page 81... ...
Best Practices 81 These crash contributing factors provide a means for identifying sections of roadway with horizontal curves throughout their system which, whether or not they have a history of severe crashes, may be expected to exhibit the greatest potential for severe crashes, especially roadway departure crashes. This empowers the agency to proactively work to reduce crashes and save lives on its rural roadway network.
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82 Guide for Quantitative Approaches to Systemic Safety Analysis 5.3.1 Trend Analysis The most common approach to evaluating a systemic safety management program is to use trend analysis. This approach evaluates a program by computing the aggregate number of target crashes that occur on the system under consideration and tracking changes to the crash frequency before, during, and after deployment of a strategy.
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Best Practices 83 excluded from the analysis. Once aggregate crash frequencies are computed, they can either be directly compared or they can be normalized.
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84 Guide for Quantitative Approaches to Systemic Safety Analysis KYTC regularly utilizes the simple before-after study evaluation approach and other evaluation approaches [see parts 5.3.3, Shift of Proportions Method and 5.3.4, Empirical Bayes (EB) Before-After Method below]
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Best Practices 85 5.3.3 Shift of Proportions Method The shift of proportions method is a common approach for evaluating the effectiveness of systemic safety management programs and treatments when traffic is expected to be an influencing factor, but traffic data are not available. It is also used to supplement other analysis methods, offering additional insight in the evaluation processes.
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86 Guide for Quantitative Approaches to Systemic Safety Analysis and may require additional, application-specific training. For this reason, other evaluation methods are often selected, with agencies indicating that they plan to work towards utilizing the EB approach in future evaluations.
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Best Practices 87 results of the evaluation show consistent reductions in observed crashes of all severities and all types relative to expected crashes. With the CMF development using the EB before-after study approach, IDOT was able to achieve an effective evaluation of its systemic programming.
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