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Accident Modification Factors for Traffic Engineering and ITS Improvements (2008)

Chapter: Chapter 2- Status of Existing AMFs and Identification of AMF Needs

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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
Page 11
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
Page 13
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
Page 14
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
Page 15
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
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Suggested Citation:"Chapter 2- Status of Existing AMFs and Identification of AMF Needs." National Academies of Sciences, Engineering, and Medicine. 2008. Accident Modification Factors for Traffic Engineering and ITS Improvements. Washington, DC: The National Academies Press. doi: 10.17226/13899.
×
Page 17

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9This chapter includes a description of the processes used to develop an initial list of important traffic engineering and ITS treatments, the process and criteria used to rate the quality of the AMFs discovered for these treatments, details of the AMFs that were judged to be of high or medium-high quality and thus included in NCHRP Research Results Digest 299 (5), and the processes used to prioritize and select other treatments for additional analyses and AMF development. Extracting Information on Existing AMFs and Determining AMF Quality Determining the Treatments to Be Considered Numerous treatments are used by state and local safety agencies in their efforts to reduce the number and severity of crashes at intersection and non-intersection (i.e., segment) locations. The NCHRP Project 17-25 team developed an initial list of 78 such treatments, categorized as intersection- related, segment-related, ITS-related, other, and combined treatments. This list was based on treatments proposed in past safety guidance documents such as the NCHRP Report 500 guides for implementation of the AASHTO Strategic Highway Safety Plan (6, 4) and on project team knowledge of state practices and knowledge of past treatment evaluations. As part of this project, an AMF state-of-the-practice sur- vey was sent to each of the 50 state DOTs. Working with FHWA, the project team identified the safety engineer and an ITS engineer in each DOT and attempted to reach them with the survey. Responses were ultimately received from 34 states. Respondents were asked to list additional treat- ments for which an AMF is needed, which expanded the list of treatments to be reviewed to 113. After some further screening based on the literature review process and collaps- ing of redundant treatments, the final list included 100 treatments. Literature Review Process The project team then searched for evaluation reports for each of these treatments in a variety of sources. These included the reference lists of the NCHRP Report 500 guides (6), docu- ments identified through the Transportation Research Infor- mation Service (TRIS) and other resource search engines, and references being reviewed in developmental work for the HSM. Given the broad scope of this project effort—traffic engineer- ing and ITS improvements—the challenge was to quickly iden- tify the most relevant studies for each treatment that required a more thorough review. Given that the focus of this effort is on AMFs, the initial screening criterion applied to each study was that the results must be founded on a crash-based analysis. Studies based on traffic behavior, survey results, or other outcomes were eliminated from consideration. The studies for each treatment were further screened to determine which ones included the development of AMFs or a methodology that could be used to develop AMFs. The studies meeting this criterion and believed to be the most credible were then subjected to a more critical review. Each critical review was undertaken with the following objectives: • Evaluate the research approach and statistical methodol- ogy, including an examination of possible pitfalls such as regression to the mean or site-selection bias. • Document the magnitude and assess the quality of any AMFs produced. • Determine whether and how each particular study could be used in Phase II of the research effort, in which addi- tional research on critical treatments was to be conducted. The three possible uses were the following: – Source data for the study could be available for further analysis using a more rigorous methodology. – Study results (and/or source data) could be used as part of a meta-analysis. – Study results could be provided to an analysis-driven expert panel. C H A P T E R 2 Status of Existing AMFs and Identification of AMF Needs

Levels of AMF Quality As noted in the second point above, the output of this effort was to be not only the documentation of the AMF, the level of effectiveness of the treatment, but also the development of a measure of the quality of the AMF—a measure of its level of predictive certainty (LOPC). This estimate of the LOPC is a reflection of the study methodology used to define the AMF. The LOPCs used in this review and the criteria for each are as follows: • High. The AMF was developed in a rigorous before-after study that incorporated the current best study design and statistical analysis methods. At this time, the empirical Bayes (EB) methodology described by Hauer represents the best available approach (7). The study must also have included a sufficiently large number of treatment sites, a large reference group composed of comparable sites, and enough crashes for statistical validity. • Medium-High. The AMF was developed in an EB before- after study with limited numbers of treatment sites and/or crashes or a before-after study that incorporated sound (but not EB) statistical methods and/or may have been reviewed and “vetted” by an expert panel of researchers. This level may include AMFs that were produced by an ex- pert research panel from the combination of findings from different (less-controlled) before-after and cross-sectional studies. The panel’s judgment concerning the quality of the AMF is reflected in the LOPC and did not always merit a medium-high rating. This level also includes AMFs that were developed in a rigorous meta-analysis by a recognized meta-analysis expert. (Meta-analysis is the combination of the results of various studies using statistical techniques that allow the expert to overcome some of the shortcom- ings of the original research.) Not all meta-analysis results warranted a medium-high LOPC (see discussion below). • Medium-Low. The AMF was developed from a cross- sectional analysis (controlling for other factors statistically) or from less-than-rigorous before-after studies, but is still judged to be of value. An example would be a before-after study in which regression to the mean was not viewed as a major potential bias because the population of treated sites included more than just “high-accident locations.” • Low. The AMF was developed in a simple before-after study without control for biases or from cross-sectional studies in which modeling techniques are questionable. • Non-existent. No studies were found that included AMFs for this treatment. It is also important to understand that within each LOPC, there can be a wide range of accuracy or confidence. For example, some AMFs have been developed in cases where the expert panel was able to utilize the results of at least one key study that was considered critical and very well done. However, in other cases, the expert panel may not have been able to identify any studies without flaws and may have been forced to rely on their collective knowledge, experience, and judgment in combining the results of these less-valid studies. It is obvious that the AMFs developed without the results from any critically valid studies have a lower level of predic- tive certainty than the ones developed with at least one such study. The criteria listed above indicate that an AMF produced from a rigorous meta-analysis may be considered to have a LOPC of medium-high. Many of the meta-analyses that have been conducted include studies from multiple countries as well as studies that are several decades old. There are enough differences among the applications of treatments in North America, Europe, and Australia to warrant caution in com- bining the results of these studies. There have also been enough changes in drivers and vehicles to warrant caution in the inclusion of studies that are more than 25 years old. Thus, the following criteria were used in assessing whether an AMF from a meta-analysis was deemed to be of medium-high quality: • A minimum of three North American studies (post-1980) had to be included in the analysis, and the percentage of North American studies had to be at least 20 percent. There was an exception to this threshold if the treatment was believed to produce operational characteristics in the United States that were different than the operational char- acteristics produced in other countries. For example, road shoulders in many European countries are designed and used for passing maneuvers, which is completely different from the function of road shoulders in the United States. In cases in which the treatment was believed to produce operational characteristics in the United States that were different than the operational characteristics produced in other countries, the percentage of studies from the United States needed to be substantially greater than 20 percent. • The treatment in the meta-analysis had to be clearly defined to be sure the results were applicable to the “treatment of interest,” including specifics on applicability to various classes of roadways. For example, if the treat- ment of interest was speed limit reduction on two-lane rural roads, then a meta-analysis in which the only U.S. studies were related to Interstates was not considered. • The results had to be statistically significant (i.e., the 95-percent confidence interval could not include 0). The intent here was to avoid including any AMF for which the sign may change (i.e., the lower end of the confidence interval results in a crash decrease while the upper end results in a crash increase). 10

Results of the Literature Review Of the 100 treatments examined, the existing research lit- erature allowed the team to conduct detailed critical reviews on 50 treatments. The information derived from these criti- cal reviews was used to summarize existing knowledge and prioritize research for Phase II of this study. First, the infor- mation was included in an “AMF Knowledge Matrix” that provides a status report on the quality of AMFs for these 100 treatments, as indicated by checkmarks within the cells in Table 1. In addition to the measure of AMF quality, the ma- trix also provides information on the user priority level for the 25 highest rated treatments and information on other ongoing or planned research that would potentially increase the quality of the AMF. The cells that are shaded within the matrix represent the top 25 treatments as rated by the state DOT respondents. The rank-order of these top 25 treat- ments is shown in the column labeled “User Priority Level.” (Note that Table 1 reflects AMF knowledge through 2004. Additional AMFs have since been developed in this and other projects and are included in the final AMF listing in Chapter 5 of this report.) Of the 50 treatments critically reviewed (and the 100 treat- ments considered), 20 were judged to have a high or medium-high LOPC. These treatments are summarized in Table 2. The asterisks in Table 2 denote the treatments in the top quartile of the user ratings. Thus, 11 of the 20 treatments deemed to have AMFs of acceptable quality were in the users’ top 25. Summary information for each of these 20 AMFs along with the knowledge matrix were published in NCHRP Research Results Digest 299 (5) as an interim product of this research study. Each summary includes the AMF(s), the LOPC, the study methodology, a description of the sites used in the study, and supplemental comments and footnotes to describe the study results and applicability. These same resulting AMFs are presented in Chapter 5 of this report along with new AMFs produced by the Phase II efforts of this research. Prioritizing Phase II Efforts to Develop Additional AMFs The results of the literature review and the input from DOT practitioners clearly supported the need for additional research to develop new AMFs and to strengthen those with less than a medium-high LOPC. Only 20 of the 100 treatments being studied had AMFs of high or medium-high quality, meaning that 80 had lower quality AMFs (or didn’t have them at all), in- cluding 14 of the DOT users’ top-25 treatments. Given project funding limitations, a decision was made concerning which treatments should be further researched in the Phase II efforts. This decision was based on the following factors: • User Priority Level. The state DOT survey respondents rated each treatment in terms of how important it was to have an AMF. These ratings were combined to provide the user priority ranking. Particular attention was given to the top quartile of these ranked treatments. • Level of Predictive Certainty. As described above, for each treatment where a critical literature review was possible, an LOPC was assigned—high, medium-high, medium-low, or low. Any treatment for which no prior research study was discovered was categorized under “non-existent.” Phase II efforts concentrated on the lower three levels. • Ongoing/Future Research. Determination of whether there was ongoing or planned research that might improve the AMF was based on a review of several research-in-progress databases, discussions with other highway safety researchers, and conversations with research sponsors such as FHWA and the Insurance Institute for Highway Safety (IIHS). The studies referenced in Table 1 are those that have the greatest potential for producing AMFs for specific treatments. Results from these studies should be reviewed in the future to determine if the LOPC for an AMF has been improved. • Estimate of Crash-Related Harm Possibly Affected by the Treatment. The importance of a treatment, and thus its AMF, is a function of the size of the safety problem that the treatment affects and the probability that the treatment will be implemented. For those treatments ranked high in terms of user priorities, it was assumed that implementation would be widespread given a sound AMF. Each of these high-priority treatments was assigned a high, medium-high, medium-low, or low crash-harm rating. This was done by assigning a target crash type to the treatment—the crash type or types that would be most affected—and defining the appropriate rating based on the economic level of national “crash harm” associated with that crash type. The economic estimates for each of 31 crash types were based on work by Miller (71). A more detailed discussion of this methodology is presented in Appendix A. • Availability of Needed Research Data. For each treat- ment being considered, detailed historic data concerning treatment descriptions and treatment dates from the implementing agencies were necessary, as well as linkable historic crash, roadway inventory, and traffic flow data for both treated sites and for comparison/reference sites. The data either had to be available from the implement- ing jurisdictions or from FHWA’s Highway Safety Infor- mation System (HSIS), which includes such historic data for nine states. The first four of these factors were then captured for a subset of the treatments shown in the AMF knowledge matrix—those that were in the users’ top 25 rankings and additional treat- ments of interest to the project oversight panel (see Table 3). 11

12 Level of Predictive Certainty Treatment User Priority Level High Medium-High Medium- Low Low Non- Existent Ongoing/ Future Work Intersection Treatments Install a roundabout 19 (8)1 Reduce or eliminate intersection skew (9) Correct sight distance (9) Install offset T’s Install turn lane or bypass lane at T-intersection (10, 11) Add exclusive left-turn lane 1 (12) Install double left-turn lane (change from single) Create positive offset for opposing left-turn lanes (13) (14) Add exclusive right-turn lane 4 (12) Add channelization for right-turns 11 (15, 16) Install median acceleration lane Add raised/painted median islands (12,17) Install a traffic signal 2 (18)2 Remove a traffic signal (19)3 Add a left-turn phase (protected or protected/permissive) 8 (20, 21) Modify signal change interval (22) Add all-red phase Change cycle length Change from incandescent to LED signals Add signal heads (23) Increase signal head size (24) Add backplates Install red-light cameras (25,26) Install red-light hold systems (27)4 Install dynamic advance warning flashers “Red Signal Ahead” 17 (28) Install overhead flashing beacon (29) Convert to all-way stop (30) Remove all-way stop Convert stop-control to yield-control (31) Prohibit left turns Install rumble strips on approach to intersection (32,33) Install intersection lighting 13 (34,35,36) (37) Close driveways near intersections 22 Install marked crosswalk (38)5 Add pedestrian signals or pedestrian phase (39,40) Install curb extensions (bulbouts) Install raised crosswalks Install raised/tabled intersection Reduce turn radius (shorten pedestrian crossing) Remove parking near intersection (41) Roadway Segment Treatments Add a travel lane 8 (42) Convert two-lane road to multilane road Reduce number of lanes (road diet) (43,44)6 Narrow lane widths to add lanes (45)7 Narrow urban lanes to install turn lane Add two-way left-turn lane (TWLTL) (9)8 (46) Replace a TWLTL with median/left-turn bays Add passing lanes (two-lane roads) (9) (47) Widen median (48,49) Install raised median 20 (8,9,45) Increase lane width 21 (9,51)9 Change shoulder width and/or type 15 (9)9 Flatten horizontal curve 12 (9,52)10 Improve curve superelevation 18 (9) Reduce grade (9) Flatten vertical curve Add static curve warning signs and/or pavement markings Add dynamic curve warning sign Table 1. AMF knowledge matrix showing AMF quality, user priority, and ongoing or future research for 100 intersection, roadway segment, and miscellaneous treatments.

13 Flatten sideslope 22 ( 59 ) ( 52 ) Install/upgrade guardrail 22 ( 60 ) Install median barriers 6 ( 60 ) ( 61 ) 13 Relocate utility poles ( 62 ) Use shoulder on freeways/expressways for bus lane Remove parking Eliminate left-turns at driveways 16 Add delineation Install roadway segment lighting 23 ( 63 ) Use dynamic message sign Use variable speed limit ( 64 ) Use automated speed enforcement Install reversible roadways/lane control Reduce speed limit ( 60 ) ( 65 , 66 ) Use differential speed limit Add sidewalk/walkway Stripe bicycle lane Add midblock pedestrian signal Install raised crosswalks (non-intersection) Install mid-block pedestrian crossing Level of Predictive Certainty Treatment User Priority Level High Medium- High Medium- Low Lo w Non- Existen t Ongoing/ Future Work Add shoulder rumble strips 5 ( 53 ) 11 ( 54 , 55 ) Add edgeline rumble strips 14 ( 56 ) 12 ( 14 ) Add centerline rumble strips (two-lane roads) 10 ( 57 ) ( 54 , 14 ) Remove roadside obstacle 3 ( 9 ) ( 58 ) Miscellaneous Treatments Lengthen acceleration lane Consolidate driveways ( 9 ) 14 Traffic calming Provide signal coordination 7 ( 67 ) Increase pavement friction ( 68 ) Provide pedestrian refuge ( 69 , 70 ) Install raised medians at crosswalk ( 39 ) Install pedestrian countdown signals Install crosswalk in-pavement lighting Install automatic pedestrian detectors Fog/wind/weather detection and warning systems Install ramp metering Use safety service patrols Implement 511/traveler information Implement integrated public safety/transportation communications Use drone radars Install truck rollover warning system Install truck height warning 1 Numbers in parentheses refer to the references for the best available AMF(s) or the ongoing/planned research effort(s). 2 AMF was developed from urban intersection dataset; no AMF exists for rural intersections. 3 AMF is for one-way streets in an urban environment. 4 Yellow-light-hold study. 5 For unsignalized intersections only; no AMF for signalized intersections. 6 There have been two studies recently conducted using different methodologies that have arrived at different conclusions regarding the magnitude of the safety effect. Both were reanalyzed as part of this research study; see results in Chapter 3. 7 Freeways only, no AMFs for other road types. 8 Information available for two-lane roads only. 9 AMFs available for rural two-lane and multilane roads; no AMF available for urban/suburban arterials. 10 AMF available for rural two-lane roads; no AMF available for rural multilane or urban/suburban arterials. 11 AMF available for freeways only; no AMF available for other road classes. 12 There has been some work on profiled pavement markings that have some similarities to edge-line rumble strips (71). 13 The project team is aware of a TRB paper under review which should increase the knowledge level significantly. 14 AMF available for rural two-lane roads only; no AMF available for rural multilane roads or urban/suburban arterials. Table 1. (Continued).

These four factors were combined by the project team into an overall ranking (high, medium-high, medium-low, or low) for possible additional research as shown in the last col- umn of Table 3. It is noted that even though only a limited number of these higher-ranked treatments could be further researched in Phase II of this project effort, the rankings in this table could also be used in future decisions concerning funding for treatment evaluations. For those treatments with a high or medium-high ranking for possible additional research, the project team then com- pleted their exploration of possible available data (the final factor in the above list of decision factors). As noted above, data requirements for a sound evaluation (such as an EB before-after analysis) included both treatment information (i.e., treatment details, date of installation, and location) and historic crash, roadway inventory, and traffic data before and after the treatment installation date for both the treatment sites and for comparable reference sites. The latter require- ment is most difficult to meet. State DOTs often have details of treatments and sometimes have conducted a simple before- after study, which means that before-treatment and after- treatment crash counts were documented. State DOTs are less likely to have retained information on traffic volume changes during the before-treatment and after-treatment periods for the treatment site. While some states do retain historic traffic volume, average annual daily traffic (AADT) data, virtually no state retains historic files of roadway inventories. The latter is necessary to identify segments of roadways or intersections that were similar to the treated locations, but were untreated during the before-treatment and after-treatment periods. Such historical data are available in HSIS for nine states. Use of those data would require that the treatment being consid- ered was implemented in one of those nine states and that treatment details can be found. As part of the user survey, the project team also asked states to provide a listing of treat- ment evaluations they had conducted in the past. If a state had evaluated one of the higher-ranked treatments of interest, follow-up conversations were held to determine if historic crash, inventory, and traffic data were available for both treat- ment sites and possible comparison/reference sites. Based on information about available data from both sources, the proj- ect team then developed a listing of proposed AMF develop- ment strategies for each treatment in Table 3 for which the overall priority for future work was high or medium-high. Table 4 shows the AMFs proposed for development or im- provement and presents suggested study methodologies for each—an EB evaluation based on new data, an analysis- driven expert panel, or a reanalysis of prior study data. Based on their review of these suggestions and on a series of follow-up discussions with the project team, the oversight panel recommended that priority should be given to conduct- ing as many EB evaluations of these high-priority treatments as possible, that expert panels were acceptable as long as they were analysis-driven (i.e., based on review of past research findings with limited additional analyses where needed), and that the project efforts should continue to be closely coordinated with ongoing work to develop safety prediction tools for urban/ suburban arterials (NCHRP Project 17-26) and rural multilane highways (NCHRP Project 17-29) for use in the HSM. Based on the funding available and on team knowledge of available data, it was decided that EB analyses would be conducted to develop AMFs for the following high-priority treatments: • Installation of a traffic signal at a rural intersection (new EB before-after evaluation); • Conversion of an undivided four-lane road to three lanes including a two-way left-turn lane (TWLTL)—a “road diet” (reanalysis of data from two previous studies); • Increasing pavement friction on intersection approaches (reanalysis of previous study data); and • Increasing pavement friction on roadway segments (reanalysis of previous study data). In addition, in order to maximize the number of AMFs produced through EB evaluations, project staff conducted a detailed analysis of data provided by Winston-Salem, North Carolina. There, the Director of Transportation had docu- mented installation records for over 70 treatment types implemented since the 1980s. There were multiple sites for 14 Treatment Level of Predictive Certainty Intersection Treatments *Install a roundabout High *Add exclusive left-turn lane High *Add exclusive right-turn lane High *Install an urban traffic signal High Remove an urban traffic signal High Modify signal change interval Medium-High Convert to all-way stop control Medium-High Convert stop-control to yield-control Medium-High Install red-light cameras High Roadway Segment Treatments Narrow lane widths to add lanes Medium-High Add passing lanes (two-lane roads) Medium-High Add two-way left-turn lane (TWLTL) Medium-High *Increase lane width Medium-High *Change shoulder width and/or type Medium-High *Flatten horizontal curve Medium-High *Improve curve superelevation Medium-High *Add shoulder rumble strips on freeways Medium-High *Add centerline rumble strips Medium-High *Install/upgrade guardrail Medium-High Miscellaneous Treatments Install raised medians at crosswalks Medium-High *Treatments in top quartile of the DOT users’ priority listing. Table 2. Treatments with AMFs that have a high or medium-high level of predictive certainty.

15 Treatments User Priority Ranking Level of Predictive Certainty Ongoing/ Future Work1 Crash Harm Rating2, 3 Overall Priority for Future Work2 Intersection Treatments Install a roundabout 19 Medium High H (4,5) L Reduce or eliminate intersection skew Medium Low ML* (4) ML Install offset T’s Non-Existent ML* (5) ML Add exclusive left-turn lane 1 High MH (10) L Create positive offset for opposing left- turn lanes Non-Existent H (5) ML Add exclusive right-turn lane 4 High L* (27) L Add channelization for right-turns 11 Non-Existent L (27) MH Install a traffic signal 2 High (urban) None (rural) H (4) L (urban) H (rural) Add a left-turn phase (protected or protected/permissive) 8 Medium Low H (5) MH Install dynamic advance warning flashers “Red Signal Ahead” 17 Low MH (10) MH Install overhead flashing beacon Non-Existent H (4) ML Convert to all-way stop Medium High H (4) ML Prohibit left turns Non-Existent MH* (5) MH Install intersection lighting 13 Low MH* (4,9) ML Close driveways near intersections 22 Non-Existent ML (16) ML Install marked crosswalk Medium Low L* (7) L Road Segment Treatments Add a travel lane 8 Low MH (11) MH Reduce number of lanes (road diet) Medium Low MH* (1) MH Add passing lanes (two-lane roads) Medium High H (6) L Widen median Low ML* (11,6) MH Install raised median 20 Non-Existent L* (6) L Increase lane width (two-lane) 21 Medium High H (6) L Increase lane width (multilane) 21 Non-Existent L (6) L Change shoulder width and/or type (two-lane) 15 Medium High H (2,3) L Change shoulder width and/or type (multilane) 15 Non-Existent L* (2,3) ML Flatten horizontal curve (two lane) 12 Medium High H (2,3) L Flatten horizontal curve (multilane) 12 Non-Existent L* (2,3) ML Improve curve superelevation 18 Medium High H (2,3) ML Add static curve warning signs and/or pavement markings Non-Existent H (2,3) H Add shoulder rumble strips (freeways) 5 Medium High H1 (2,3)2 L Add shoulder rumble strips (multi-lane divided) 5 Medium Low L* (2,3) ML Add shoulder rumble strips (two lane) 5 Non-Existent H (2,3) MH Add edgeline rumble strips 14 Non-Existent H (2,3) MH Add centerline rumble strips (two-lane roads) 10 Medium High H (6) L Remove roadside obstacle 3 Medium Low H (2) MH Flatten sideslope 22 Low H (3) MH Install/upgrade guardrail 22 Medium High H (2,3) ML Install median barriers 6 Medium Low L* (6) L Eliminate left turns at driveways 16 Non-Existent ML (16) MH Install roadway segment lighting 23 Medium Low MH MH Add sidewalk/walkway Non-Existent ML* (1) ML Add midblock pedestrian signal Non-Existent MH* (1) MH Install raised crosswalks (non-intersection) Non-Existent MH* (1) MH Install mid-block pedestrian crossing Non-Existent MH* (1) ML Miscellaneous Treatments Consolidate driveways Medium Low MH (10,20) ML Provide signal coordination 7 Non-Existent MH (10,12) MH Increase pavement friction Low MH* (2,3) MH Install crosswalk in-pavement lighting Non-Existent H (1) ML 1Checkmarks reflect treatments for which there is ongoing or planned future work to develop AMFs. 2H = High, MH = Medium-High, ML = Medium-Low, L = Low. 3Primary crash types are shown in parentheses with the crash harm rating. See Appendix A for discussion of the crash types. Those ratings that were adjusted are designated with an asterisk. Table 3. Factors used and final rankings for additional research on intersection, segment, and miscellaneous treatments.

most of the treatment types. He had consistently conducted simple before-after studies of the effects of these treatments on both total crashes and “target” crashes (e.g., angle crashes for stop-to-signal conversions). These studies usu- ally contained 3 to 5 years of both before-treatment and after-treatment crash data. The documented data did not contain AADT data across the study years, and there were no computerized data files that would allow the develop- ment of a reference group for use in EB analyses. However, if these data could be obtained from other sources, then, if carefully chosen, multiple treatments could be analyzed using the same reference group. The project team then examined the data to identify treatment types evaluated in recent years that had sufficient sample sizes in the before- treatment and after-treatment periods to allow statistical significance tests. The team also identified “clean” treat- ment sites with each treatment type, where no additional treatments had been applied during the before-treatment and after-treatment periods (a major undertaking since many of the sites had undergone more than one treatment). Based on these preliminary analyses, a decision was made to evaluate the following treatments at signalized intersections: • Modification of a left-turn signal phase (three combinations), • Replacement of an 8-in. signal head with 12-in. head, • Replacement of a single red signal head with dual red signal heads, and • Conversion of nighttime flashing operation to steady oper- ation. The first of these, the left-turn phase treatment, is included in Table 4. While the latter three treatments are not included as high-priority treatments in that table, examination of ear- lier studies in the Phase I literature review indicated that the AMFs existing for the second and third treatments (replacing 8-in. signal head with 12-in. head and replacing single red signal head with dual red signal heads) were rated as medium- low in predictive certainty. The final one (converting night- time flashing operation to steady operation) had no AMF. In addition, this work allowed the researchers to develop AMFs for treatments of high interest to local (urban) traffic engi- neers. Finally, as noted above, the use of a single reference group greatly lowered the evaluation cost per treatment. The other approach to AMF development/modification involved two analysis-driven expert panels. While earlier project discussions and information in Table 4 suggested the possibility of expert panels for AMFs related to specific focus areas (e.g., roadside crashes and pedestrian treatments), it was decided by the team and the oversight panel that a more critical need was to assist the research teams for NCHRP 16 Possible Study Methodologies High Priority Treatments (Grey shading indicates high ranking in user survey) Overall Priority for Future Work EB Evaluation Expert Panel Reanalysis of Prior Study Data Intersection AMFs Install or remove a signal H (Rural) Add a left-turn phase (permissive/protected or protected-only) MH Channelize right turns MH Install dynamic advance warning flashers “Red Signal Ahead” MH Provide signal coordination MH Prohibit left turns MH Increase pavement friction on approaches H Segment AMFs Add a travel lane MH Remove roadside obstacles (including urban) MH Add shoulder rumble strips (two- lane/others) MH Add edgeline rumble strips MH Eliminate left turns at driveways MH Flatten sideslopes MH Install roadway segment lighting MH Add advance curve warning signs/on- pavement markings H Increase pavement friction MH Add midblock pedestrian signal MH Raise crosswalks (non-intersection) MH Reduce number of lanes (road diet) MH Widen median MH *H = High, MH = Medium-High, ML = Medium-Low, L = Low. Table 4. Possible study methodologies for high-priority AMFs.

Projects 17-26 and 17-29 in developing AMFs for the safety prediction tools they were developing for urban/suburban arterials and rural multilane highways. Because of the large number of serious and fatal crashes that occur on two-lane rural roads, much of the past AMF development work had focused on treatments for that roadway class. Literature on AMFs for other roadway classes is limited. It was hoped that expert panels might be able to combine the limited past eval- uations specific to these two roadway classes with modified versions of two-lane AMFs to develop the needed estimates. It was also hoped that this option might produce multiple AMFs at a cost lower than the cost of new analyses. The results of these efforts are presented in Chapter 4. Summary The Phase I efforts of this research study identified 100 treatments that are used by state and local DOTs, ranked these treatments based on inputs from a state DOT user sur- vey, and produced documentation of 20 AMFs that were judged to be of high or medium-high quality from a critical review of existing research literature. This effort was closely coordinated with other ongoing NCHRP efforts to docu- ment AMFs for the upcoming publication of the HSM. Indeed, the criteria used to make this judgment of AMF quality were the basis for similar AMF reviews in the HSM project. The project team then developed a unique process for identifying high-priority AMF research needs that incor- porates the priorities of DOT practitioners, the current level of AMF quality, knowledge of other ongoing or future re- search that might enhance certain treatment AMFs, and a measure of the effect on crash harm that a specific treatment might have if implemented widely. For those treatments that were ranked highest in terms of need by this procedure, the team explored available state and national databases and proposed a research strategy to develop or improve the AMF for each treatment. Working with the project oversight panel, a final list of research tasks for the Phase II effort was developed. The research conducted is described in the next two chapters of this report. 17

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TRB's National Cooperative Highway Research Program (NCHRP) Report 617: Accident Modification Factors for Traffic Engineering and ITS Improvements explores the development of accident modification factors (AMFs) for traffic engineering and intelligent transportation system improvements. AMFs, also known as crash reduction factors, are designed to provide a simple and quick way of estimating the safety impacts of various types of engineering improvements, encompassing the areas of signing, alignment, channelization, and other traffic engineering solutions.

The following appendices to NCHRP Report 617 are available online:

* Appendix A: Methodology for Determining Crash-Harm Rating for Treatments

* Appendix B: Effects of Converting Rural Intersections from Stop to Signal Control

* Appendix C: Safety Effects of Four-Lane to Three-Lane Conversions

* Appendix D: Safety Effects of Improving Pavement Skid Resistance

* Appendix E: Evaluation of the Safety Effectiveness of Urban Signal Treatments

* Appendix F: An Empirical Examination of the Relationship Between Speed and Road Accidents

* Appendix G: Accident Modification Factors for Median Width

Two AMF treatment summaries that appear in the printed version of NCHRP Report 617 contain incorrect information. These treatments are "Add Intersection Lighting" and "Add Roadway Segment Lighting." The information in these two AMF treatment summaries has been corrected in the online version of the report.

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