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From page 6... ... 6 This chapter presents the research conducted under each of the three major tasks presented above. Each task follows a consistent format in the use of subtasks. Read the entire page →
From page 7... ... Findings and Applications 7 function of site characteristics. A user guide for the CMF Regression Software is presented in Appendix D Read the entire page →
From page 8... ... 8 Guidelines for the Development and Application of Crash Modification Factors The OECD report argues that to assess the international transferability of CMFs from evaluation studies, many studies should have been reported in many countries during a long period, and the study findings should not vary systematically between countries or over time. The report presents an approach for determining whether the study findings vary systematically between countries over time. Read the entire page →
From page 9... ... Findings and Applications 9 limited analysis of the effect of flashers, speed limits, and major and minor road annual average daily traffic (AADT) Read the entire page →
From page 10... ... 10 Guidelines for the Development and Application of Crash Modification Factors • Signal to roundabout conversion. A recent study (Srinivasan et al. Read the entire page →
From page 11... ... Findings and Applications 11 developing the guidelines, the team postulated that differences in the crash type and severity distributions between the two sites (site where CMF was developed and site where it is intended to be applied) were the priority for adjustment. Read the entire page →
From page 12... ... 12 Guidelines for the Development and Application of Crash Modification Factors than converting a four-lane road to a six-lane road. The number of lanes is not simply a CMF influential factor, but rather, it is integrated as part of the countermeasure itself. Read the entire page →
From page 13... ... Findings and Applications 13 • Development of Adjustment Functions to Assess Combined Safety Effects of Multiple Treatments on Rural Two-Lane Highways (Park and Abdel-Aty 2015) • Exploration and Comparison of Crash Modification Factors for Multiple Treatments on Rural Multilane Roadways (Park et al. Read the entire page →
From page 14... ... 14 Guidelines for the Development and Application of Crash Modification Factors that number of intersection legs is an influential factor in the effectiveness of converting a traffic signal to a roundabout. The team conducted similar homogeneity tests for countermeasures for which there were sufficient disaggregate CMFs to allow for matching on all site characteristics except the one of interest. Read the entire page →
From page 15... ... Findings and Applications 15 identify state-to-state differences in CMFs. These analyses were conducted on data sets obtained for the evaluation of three countermeasures • Centerline and shoulder rumble strips • Reflective pavement markings on edgelines and lane lines (freeways) Read the entire page →
From page 16... ... 16 Guidelines for the Development and Application of Crash Modification Factors as AADT and expected crashes in the before period, were found to be influential factors. The details of these analyses are provided in Appendix A, Section A.4. Read the entire page →
From page 17... ... Findings and Applications 17 Subtask 2.6c Converting Aggregate CMFs to Specific Crash Type and Severity Many geometric and traffic control elements associated with a roadway facility influence its safety (as defined by its crash frequency or severity distribution) Read the entire page →
From page 18... ... 18 Guidelines for the Development and Application of Crash Modification Factors where CMF1r = CMF for the effect of lane width on total crashes of all types and severities CMFra = CMF for the effect of lane width on related crashes (single-vehicle run-off-road, head-on, opposite-direction sideswipe, and same-direction sideswipe) pra = proportion of total crashes that are related An important point to note regarding Equation 2 is that the value of CMF1r will vary with the value of the proportion pra. Read the entire page →
From page 19... ... Findings and Applications 19 Florida values. A closed-form solution for the two disaggregate CMF values emerges from these two equations, given the number of study locations and equation variables. Read the entire page →
From page 20... ... 20 Guidelines for the Development and Application of Crash Modification Factors Area Type Roadway Type Median State Crash Type Severity CMF Urban Freeway Divided Pennsylvania All All 0.9862 Rural Freeway Divided Missouri All All 1.0789 Pennsylvania All All 1.0033 Multilane Divided Minnesota All All 1.1022 Missouri All All 1.2200 Pennsylvania All All 0.8671 2-lane Undivided Minnesota All All 1.1438 Missouri All All 1.4049 Pennsylvania All All 0.7560 Urban Freeway Divided Pennsylvania All Fatal-andInjury 0.8399 Rural Freeway Divided Missouri All Fatal-andInjury 0.9416 Pennsylvania All Fatal-andInjury 0.8739 Multilane Divided Minnesota All Fatal-andInjury 0.7779 Missouri All Fatal-andInjury 0.9475 Pennsylvania All Fatal-andInjury 0.5988 2-lane Undivided Minnesota All Fatal-andInjury 1.0513 Missouri All Fatal-andInjury 0.8076 Pennsylvania All Fatal-andInjury 0.8203 Urban Freeway Divided Pennsylvania SV-ROR All 0.9419 Rural Freeway Divided Missouri SV-ROR All 0.9209 Pennsylvania SV-ROR All 0.8229 Multilane Divided Minnesota SV-ROR All 1.3836 Missouri SV-ROR All 1.4478 Pennsylvania SV-ROR All 0.7454 2-lane Undivided Minnesota SV-ROR All 1.1072 Missouri SV-ROR All 1.1687 Pennsylvania SV-ROR All 0.5641 Urban Freeway Divided Pennsylvania SV-ROR Fatal-andInjury 0.9257 Rural Freeway Divided Missouri SV-ROR Fatal-andInjury 0.8436 Pennsylvania SV-ROR Fatal-andInjury 0.7680 Multilane Divided Minnesota SV-ROR Fatal-andInjury 0.8971 Missouri SV-ROR Fatal-andInjury 1.0016 Pennsylvania SV-ROR Fatal-andInjury 0.8014 2-lane Undivided Minnesota SV-ROR Fatal-andInjury 0.6759 Missouri SV-ROR Fatal-andInjury 0.5541 Pennsylvania SV-ROR Fatal-andInjury 0.6334 Abbreviation: SV-ROR, single-vehicle run-off-road crash. Table 4. Read the entire page →
From page 21... ... Findings and Applications 21 A regression model was developed to estimate the disaggregate CMFs, where the number of observations equaled the number of rows in Table 4 (i.e., 36) Read the entire page →
From page 22... ... 22 Guidelines for the Development and Application of Crash Modification Factors are undoubtedly reflected in the crash "location" distribution (i.e., distribution of crashes by travel direction or intersection leg) Read the entire page →
From page 23... ... Findings and Applications 23 values can be used with Equation 4 to "locally calibrate" a spatially aggregate CMF for the subject site in Iowa (using a representative crash distribution for the Iowa site) Read the entire page →
From page 24... ... 24 Guidelines for the Development and Application of Crash Modification Factors to represent a reasonable range of possible values for the aggregate CMF, depending on the proportion of crashes associated with the treated legs. This range is comparable to the aggregate CMF value in Column 5. Read the entire page →
From page 25... ... Findings and Applications 25 bay on one leg and the frequency of crashes within the intersection (but not on the treated leg) Read the entire page →
From page 26... ... 26 Guidelines for the Development and Application of Crash Modification Factors The product of Task 3 includes a recommended method for combining single-effect CMFs to estimate the joint effect of multiple treatments. Guidelines are provided to help determine the likelihood of overlapping effects and the appropriate method to account for the interaction among multiple treatments. Read the entire page →
From page 27... ... Findings and Applications 27 While this method reduces the effect of each subsequent treatment, it still suffers from the primary limitation of the additive effects approach (i.e., the combined effect could exceed 100%) Read the entire page →
From page 28... ... 28 Guidelines for the Development and Application of Crash Modification Factors Multiplicative with Empirical-Based Reduction of Combined Effect. This method is like the multiplicative approach with generalized reduction but reduces the total combined effect by a set amount based on empirical research conducted as part of this project. Read the entire page →
From page 29... ... Findings and Applications 29 CMF1 is applied to Nbase1 (A+B) to estimate the expected crashes after treatment (N1) Read the entire page →
From page 30... ... 30 Guidelines for the Development and Application of Crash Modification Factors variables for the treatments of interest, then a CMFunction may be derived to estimate the combined effect of those treatments. One advantage of using regression-based models is that there are many CMFs for which a before-after evaluation is seldom possible. Read the entire page →
From page 31... ... Findings and Applications 31 As such, it is important to encourage the estimation of variance during CMF development and consistent reporting of the results. The methodology also assumes that the CMFs are independent random variables, which may or may not be the case. Read the entire page →
From page 32... ... 32 Guidelines for the Development and Application of Crash Modification Factors > ∗12 1 2 Equation 26CMF CMF CMF Equation 2712 1 2< ∗CMF CMF CMF Equation 2812 1 2= ∗CMF CMF CMF where 12CMF = true safety effect of applying treatments 1 and 2 (i.e., parameter) CMF1 ∗ CMF2 = simple product of two CMFs (i.e., parameter estimate) Read the entire page →
From page 33... ... Findings and Applications 33 CMF1 ∗ CMF3 = simple product of CMF 1 and 3 (i.e., parameter estimate) CMF1 ∗ CMF2 ∗ CMF3 = simple product of CMF 1, 2, and 3 (parameter estimate) Read the entire page →
From page 34... ... 34 Guidelines for the Development and Application of Crash Modification Factors increasing other crash types and severities. Even if multiple treatments are independent, the respective CMFs may be related to different crash types and/or severities. Read the entire page →
From page 35... ... Findings and Applications 35 Subtask 3.2 Develop Framework for Guidelines The second subtask proposed a general framework for the guidelines. The following is an annotated outline of the proposed sections for the guidelines. Read the entire page →
From page 36... ... 36 Guidelines for the Development and Application of Crash Modification Factors Subtask 3.3 Identify Potential Data Sources for Phase 2 The third subtask identified data sources from which to test various existing and proposed methods. This included a review of the CMF Clearinghouse to identify existing CMFs for the effect of combined treatments. Read the entire page →
From page 37... ... Findings and Applications 37 Reducing Collisions at Signalized Intersections (Srinivasan et al. 2011; Antonucci et al. Read the entire page →
From page 38... ... 38 Guidelines for the Development and Application of Crash Modification Factors nearly 5,800 different treatments. With the states' permission, team members reviewed this database to search for locations where multiple treatments were implemented. Read the entire page →
From page 39... ... Findings and Applications 39 A detailed work plan is presented in this section to guide the efforts required to complete Phase 2. The following sections describe the proposed method to collect and assemble the required data, estimate the ground truth for specific combinations of treatments, assess the credibility of existing and proposed methods, and develop final guidelines for combining single-effect CMFs to estimate the joint effect of multiple treatments. Read the entire page →
From page 40... ... 40 Guidelines for the Development and Application of Crash Modification Factors Combined Treatment Combined Treatment ID Combined Treatment Star Rating Combined Treatment Applicability Individual Applicable CMFs Available? Individual CMF IDs (From Clearinghouse) Read the entire page →
From page 41... ... Findings and Applications 41 (continued on next page) Improve signal visibility, including signal lens size upgrade, installation of new backplates, the addition of reflective tapes to existing backplates, and installation of additional signal heads 277 4 Urban, 4legged, signalized Yes 12-inch lenses (1444) Read the entire page →
From page 42... ... 42 Guidelines for the Development and Application of Crash Modification Factors Combined Treatment Combined Treatment ID Combined Treatment Star Rating Combined Treatment Applicability Individual Applicable CMFs Available? Individual CMF IDs (From Clearinghouse) Read the entire page →
From page 43... ... Findings and Applications 43 Combined Treatment Combined Treatment ID Combined Treatment Star Rating Combined Treatment Applicability Individual Applicable CMFs Available? Individual CMF IDs (From Clearinghouse) Read the entire page →
From page 44... ... 44 Guidelines for the Development and Application of Crash Modification Factors three categories, denoting the change in safety relative to the base condition. In this case, CMFs for A2, B1, and B2 would represent the effect of Treatment Q, Treatment P, and the combined effect of the two treatments, respectively. Read the entire page →
From page 45... ... Findings and Applications 45 conditions in common or the treatment influences the same drivers at the same (or a nearby) location. Read the entire page →
From page 46... ... 46 Guidelines for the Development and Application of Crash Modification Factors Equation 38CMF CMF exp bQ P Q 3( ) = × Equation 39CMF CMF exp bP Q P 3( ) Read the entire page →
From page 47... ... Findings and Applications 47 multiplicative interaction effect that can be used with existing CMFs to estimate the combined treatment effect. Equation 43y exp b b x . Read the entire page →
From page 48... ... 48 Guidelines for the Development and Application of Crash Modification Factors Case 2, Option 1 -- Estimating the Combined Effect when Interaction Is Unknown. The objective of this method is to estimate the combined effect of multiple treatments when the interaction is unknown or cannot be quantified. Read the entire page →
From page 49... ... Findings and Applications 49 for further analysis. It is important to assess the validity of "common" methods to provide practitioners with evidence for either continuing along the current path or seeking an alternative method. Read the entire page →
From page 50... ... 50 Guidelines for the Development and Application of Crash Modification Factors Data Collection The following sections describe the data collected and assembled for each of the three combination treatments. Combination of Centerline and Shoulder Rumble Strip Installation. Read the entire page →
From page 51... ... Findings and Applications 51 Step 1: Identify Short Segments and Merge into Longer Segments. In this step, the research team identified short roadway segments and examined the possibility of combining these segments into longer segments for analysis. Read the entire page →
From page 52... ... 52 Guidelines for the Development and Application of Crash Modification Factors Segment Segment Rumble Strip Segments HSIS Combined Segments Segment 1: Segment 2-1: Segment 2-2: Figure 4. Illustration of tagging roadway segments. Read the entire page →
From page 53... ... Findings and Applications 53 screen capture from the WSDOT photo log. While the team did not confirm the treatment presence and type for all segments, they randomly checked and confirmed at least 20% of the segments. Read the entire page →
From page 54... ... 54 Guidelines for the Development and Application of Crash Modification Factors were selected for study because horizontal curve information is readily available in the HSIS database. Horizontal curve data supplement geometric characteristics and allow the research team to perform separate analyses for tangent and curve segments. Read the entire page →
From page 55... ... Findings and Applications 55 At the time of the data request, the most recent year of data available in the HSIS database was 2012. Therefore, 2006–2008 and 2008–2010 were the most recent three years of data in which the reporting threshold remained the same for Illinois and Ohio, respectively. Read the entire page →
From page 56... ... 56 Guidelines for the Development and Application of Crash Modification Factors (i.e., 11-ft lanes and 3-ft or 2-ft shoulders) Read the entire page →
From page 57... ... Findings and Applications 57 Field Data Collection. Intersections were visited during late spring, summer, and early fall (i.e., when the foliage is robust) Read the entire page →
From page 58... ... 58 Guidelines for the Development and Application of Crash Modification Factors • Lane width • Shoulder width • Posted speed limit • Presence of turn lanes Since available ISD was determined by independent field teams, the project team developed a standardized method for obtaining available ISD. First, the intersection was broken into individual approach directions, which serve as the unit of analysis. Read the entire page →
From page 59... ... Findings and Applications 59 crashes needed to be applied to a specific sightline. The following crash types were identified for analysis of individual and combined CMFs: • Total target crashes. Read the entire page →
From page 60... ... 60 Guidelines for the Development and Application of Crash Modification Factors There were too few observations with available ISD less than 500 ft and too few observations between 1,000 and 1,320 ft for further consideration. Intersection angle was considered in three levels for analysis, including the following: • 50 degrees to 75 degrees • 75 degrees to 85 degrees • 85 degrees to 90 degrees There were too few observations below 50 degrees for further consideration. Read the entire page →
From page 61... ... Findings and Applications 61 identify a suitable reference group for each treatment category. The focus crash types for this analysis included the following: • Total crashes (all crash types and severities) Read the entire page →
From page 62... ... 62 Guidelines for the Development and Application of Crash Modification Factors Applicable Crash Type CMF Standard Error Total crashes 0.994 0.063 Fatal-and-injury crashes 1.015 0.102 Run-off-road crashes 0.820 0.073 Target crashes 0.819 0.069 Table 19. CMFs for widening shoulder width to four feet on tangents. Read the entire page →
From page 63... ... Findings and Applications 63 Table 24 through Table 26 present the CMFs and standard errors for various lane and shoulder combinations on rural, two-lane, undivided horizontal curve sections. Each table presents the CMFs and standard errors for the four crash types of interest. Read the entire page →
From page 64... ... 64 Guidelines for the Development and Application of Crash Modification Factors • Right-angle crashes. Subset of total target crashes in which both vehicles on the major and minor roads were intending to move straight through the intersection. Read the entire page →
From page 65... ... Findings and Applications 65 • Multiplicative with Systematic Reduction: CMF CMF 1 1 CMF 2 t 1 2= × − −     • Dominant Effect: CMFt = CMF1 (largest effect) • Dominant Common Residuals: CMFt = (CMF1 × CMF2) Read the entire page →
From page 66... ... 66 Guidelines for the Development and Application of Crash Modification Factors Method CMF1 CMF2 Combined CMF (By method) Combined CMF (Ground truth) Read the entire page →
From page 67... ... Findings and Applications 67 the combined effect. Based on the CMFs for CLRS, SRS, and CLRS+SRS, the Multiplicative with Generalized Reduction method consistently underestimates the combined effect (4 of 4) Read the entire page →
From page 68... ... 68 Guidelines for the Development and Application of Crash Modification Factors Method CMF 1 CMF 2 Combined CMF (By method) Combined CMF (Ground truth) Read the entire page →
From page 69... ... Findings and Applications 69 total crashes, fatal-and-injury crashes, run-off-road crashes, and target crashes (run-off-road + head-on + sideswipe) , respectively. Read the entire page →
From page 70... ... 70 Guidelines for the Development and Application of Crash Modification Factors Method CMF 1 CMF 2 Combined CMF (By method) Combined CMF (Ground truth) Read the entire page →
From page 71... ... Findings and Applications 71 For horizontal curve segments, the combination scenario presented in this study is 12-ft lanes and 8-ft shoulders compared to 11-ft lanes and 2-ft shoulders. Table 44 through Table 47 present the assessment results for total crashes, fatal-and-injury crashes, run-off-road crashes, and target crashes (run-off-road + head-on + sideswipe) Read the entire page →
From page 72... ... 72 Guidelines for the Development and Application of Crash Modification Factors Method CMF1 CMF 2 Combined CMF (By method) Combined CMF (Ground truth) Read the entire page →
From page 73... ... Findings and Applications 73 Note, however, that some CMFs for total crashes are not statistically significant at the 95% confidence level. Based on the CMFs for 12-ft lanes and 8-ft shoulders (Combination 2, Table 49) Read the entire page →
From page 74... ... 74 Guidelines for the Development and Application of Crash Modification Factors Table 53 presents a summary of the difference between the ground truth and estimated combined CMF from each method for the lane-shoulder combination on horizontal curves. The difference is always computed as the ground truth minus the estimate from the respective method. Read the entire page →
From page 75... ... Findings and Applications 75 Method Difference Total Crashes Difference F&I Crashes Difference ROR Crashes Difference Target Crashes Average Difference Multiplicative = CMF1* CMF2 0.044 0.180 -0.049 -0.004 0.043 Multiplicative with Generalized Reduction = 1−(2/3* Read the entire page →
From page 76... ... 76 Guidelines for the Development and Application of Crash Modification Factors ISD and intersection angle. Note that CMF1 is always the smaller estimated individual CMF, and CMF2 is always the larger estimated individual CMF. Read the entire page →
From page 77... ... Findings and Applications 77 effect (3 of 3) Read the entire page →
From page 78... ... 78 Guidelines for the Development and Application of Crash Modification Factors for total and fatal-and-injury crashes. The Multiplicative with Systematic Reduction method performs best for run-off-road and right-angle crashes. Read the entire page →
From page 79... ... Findings and Applications 79 Table 62. Summary of average MSE by method by inclusion of CMFs > 1.0. Read the entire page →
From page 80... ... 80 Guidelines for the Development and Application of Crash Modification Factors method is not appropriate when combining CMFs greater than 1.0 because the product of the individual CMFs is then raised to a power greater than 1.0, which intensifies rather than dampens the combined effect. The following are the four specific cases listed in the table. Read the entire page →
From page 81... ... Findings and Applications 81 • For scenarios where both CMFs are less than 1.0, the Dominant Common Residuals method performs best. • For scenarios where one or more CMFs are greater than 1.0, the Dominant Effect method performs best. Read the entire page →
From page 82... ... 82 Guidelines for the Development and Application of Crash Modification Factors Additionally, the Multiplicative method severely overestimates the combined treatment effect in these three scenarios. From the equation above, it is apparent that overestimating the combined treatment effect results in underestimating the standard error of the combined treatment effect. Read the entire page →
From page 83... ... Findings and Applications 83 that two variables are perfectly correlated, while a value of 0.00 indicates no correlation between the variables. The direction of the correlation characterizes the relationship between the variables; however, the strength of correlation for 1.00 and –1.00 is the same. Read the entire page →
From page 84... ... A B C D E F G H I J K L M N O P Q R S T A 1.00 B 0.47 1.00 C 0.00 0.04 1.00 D 0.49 0.37 0.03 1.00 E 0.24 0.38 0.09 0.09 1.00 F 0.32 0.48 -0.31 0.34 0.17 1.00 G 0.32 0.50 -0.20 0.32 0.31 0.93 1.00 H -0.17 0.07 -0.01 -0.05 0.13 0.21 0.20 1.00 I -0.23 0.13 -0.03 -0.12 0.24 0.13 0.22 0.80 1.00 J 0.34 -0.02 -0.19 0.23 -0.30 0.29 0.17 -0.06 -0.16 1.00 K -0.03 0.17 -0.26 -0.16 0.01 0.31 0.21 0.16 0.00 -0.06 1.00 L -0.13 0.09 -0.21 -0.16 0.15 0.27 0.23 0.14 0.05 -0.12 0.89 1.00 M 0.35 0.04 -0.14 0.22 -0.34 0.34 0.22 -0.05 -0.12 0.89 0.04 -0.07 1.00 N -0.34 -0.10 0.01 -0.22 0.19 -0.17 -0.07 -0.06 0.04 -0.62 0.17 0.32 -0.64 1.00 O 0.41 0.04 -0.17 0.24 -0.32 0.32 0.18 -0.04 -0.16 0.95 0.05 -0.07 0.93 -0.66 1.00 P -0.41 -0.04 0.17 -0.24 0.32 -0.32 -0.18 0.04 0.16 -0.95 -0.05 0.07 -0.93 0.66 -1.00 1.00 Q -0.14 -0.19 0.16 0.01 -0.14 -0.39 -0.39 -0.14 -0.08 -0.17 -0.38 -0.45 -0.14 -0.05 -0.14 0.14 1.00 R -0.13 -0.13 0.22 0.04 -0.04 -0.32 -0.23 0.00 0.13 -0.17 -0.41 -0.40 -0.15 -0.25 -0.20 0.20 0.24 1.00 S -0.30 -0.21 0.21 -0.17 0.03 -0.45 -0.29 -0.01 0.18 -0.27 -0.30 -0.23 -0.24 -0.05 -0.31 0.31 0.10 0.64 1.00 T 0.23 0.22 -0.36 0.12 0.05 0.30 0.17 0.00 -0.17 0.17 0.37 0.25 0.11 0.19 0.15 -0.15 -0.16 -0.67 -0.63 1.00 Key A: Major-road AADT B: Minor-road AADT C: Posted speed limit D: Lane width E: Presence of stop line F: Presence of advance intersection warning sign G: Distance from intersection to advance intersection warning sign H: Presence of advance STOP AHEAD sign I: Distance from intersection to advance STOP AHEAD sign J: Presence of edgeline extension K: Presence of speed advisory sign L: Level of speed reduction from posted speed limit to advisory speed M: Average annual temperature N: Total annual snowfall O: Indicator for North Carolina P: Indicator for Ohio Q: Intersection angle R: Available ISD S: Indicator for flat terrain (base condition rolling or mountainous) T: Grade 500 ft before intersection on a major road Table 65. Read the entire page →
From page 85... ... Findings and Applications 85 Characteristics that are strongly related make it difficult to discern the individual effects when the two characteristics are considered jointly. Example 1, in Table 65, shows that the correlation coefficient between the presence of an edgeline extension and the indicator for North Carolina is 0.95. Read the entire page →
From page 86... ... 86 Guidelines for the Development and Application of Crash Modification Factors An example of this is evident in a safety effectiveness evaluation conducted under FHWA's Development of Crash Modification Factors project (Himes et al. Read the entire page →
From page 87... ... Findings and Applications 87 shoulder width on rural two-lane, undivided tangent sections. The following are the two combined treatments of interest for tangent sections. Read the entire page →
From page 88... ... 88 Guidelines for the Development and Application of Crash Modification Factors Guidelines on Combining Two CMFs to Estimate a Combined Treatment Effect Before estimating the combined safety effect of multiple treatments, the analyst must decide if it is necessary and appropriate to implement multiple treatments. If the treatments target different safety issues and crash types, then it may be necessary to implement both treatments, and the analyst can proceed with the following guidelines to consider the combined effect. Read the entire page →
From page 89... ... Findings and Applications 89 Determine Potential Overlap of Individual Treatment Effects Case A: Zero overlap Case B: Some overlap Case C: Complete overlap Case D: Enhancing effects Case E: Counteracting effects Determine Magnitude of Individual Treatment Effects Small (< 10% change) Medium (10% - 25% change) Read the entire page →
From page 90... ... 90 Guidelines for the Development and Application of Crash Modification Factors • Specific treatment(s) Read the entire page →
From page 91... ... Findings and Applications 91 The second group provides complementary discussion in two appendixes. These appendixes make the case that researchers and highway safety institutions that influence the conduct of research should engage, where practical, in randomized trials or at least in an approach that is close to achieving this desideratum. Read the entire page →
From page 92... ... 92 Guidelines for the Development and Application of Crash Modification Factors the guidelines, as well as how they can be mitigated. It was recognized that there is some research suggesting that, despite these challenges, it is possible to explore the development of CMFunctions with this approach, especially where this is a necessity and where corroboration with before-after results and intuitive reasoning is possible. Read the entire page →
From page 93... ... Findings and Applications 93 to overcome some of the key issues with observational studies that are illuminated in the recommended procedures. This guidance was designed for institutions; namely, state and local transportation agencies (at present) Read the entire page →
From page 94... ... 94 Guidelines for the Development and Application of Crash Modification Factors The data came from the three sources listed below. Some of the sites were used for both Source 1 and Source 2 and were only included once in this analysis. Read the entire page →
From page 95... ... Findings and Applications 95 provided in conjunction with the case studies, with extensive and appropriate cross-referencing. In sum, the plan then was for the case studies to complement the procedural guidelines by: • Illustrating a heuristic methodology a future researcher may follow to derive a CMFunction. Read the entire page →
From page 96... ... 96 Guidelines for the Development and Application of Crash Modification Factors – What is the best way to predict? – Integration of research and practice • Developing consensus in research about the safety effect of manipulations (Appendix G) Read the entire page →

From page 6...

... 6 This chapter presents the research conducted under each of the three major tasks presented above. Each task follows a consistent format in the use of subtasks.