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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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Suggested Citation:"Chapter 4. Gap Analysis." National Academies of Sciences, Engineering, and Medicine. 2021. Application of Crash Modification Factors for Access Management, Volume 2: Research Overview. Washington, DC: The National Academies Press. doi: 10.17226/26162.
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66 C H A P T E R 4 Gap Analysis Introduction Chapter 2 documented the results of a literature review and chapter 3 documented the results of an assessment of existing data sources. This chapter identifies areas where additional research is needed and is feasible. The purpose of the gap analysis is to focus this research effort to leverage quality existing information and build on past research to fill priority voids and/or limitations in the current state-of-the- knowledge. For example, there is an opportunity to make use of datasets used to develop SPFs and CMFs in the Highway Safety Manual (1st Edition) and supplement those datasets with additional variables to incorporate access management strategies and overcome data limitations from past studies. Summary of Gap Analysis and Possible Data Sources The gap analysis follows the structure of the literature review and focuses on the three primary levels of implementation (i.e., site, intersection, and corridor). The intent is to identify the availability of quality CMFs (i.e., those CMFs rated 4 or 5 stars in the CMF Clearinghouse) for each strategy and implementation level. If SPFs are available that could be used to infer CMFs (by dividing the ‘with’ SPF prediction by the ‘without’ SPF prediction), this is also noted. If SPFs are available, but they are not calibrated using data from the same jurisdiction and time period, the difference between predictions could be due to factors other than those related to the site characteristics. This makes it challenging to infer CMFs from SPFs and is noted as such in this table. Starting with the information in chapter 2, the project team reviewed the high-priority strategies (based on the survey described in chapter 3) to identify those where quality CMFs are not available and where estimating new CMFs may be feasible. Table 49 presents these priority strategies, indicating where CMF information is lacking and identifying potential data sources to support CMF or SPF development. Table 50 presents strategies for which knowledge gaps exist and future research may be needed but were not identified as high priority strategies from the survey. Those data sources shown in bold face in the tables were the focus of the data collection effort (Task 7). The other potential data sources were considered.

67 Table 49. Gaps identified as high priority. Strategy Sub-strategy CMF Availability Missing CMFs Possible Data Sources Manage location, spacing, and design of median openings and crossovers Create directional median opening No CMFs at 3 or more stars CMFs for corridor and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) Brigham Young University Project: Safety Analysis of Access Management Implementation in Utah Regulate median opening density 5 CMFs at 3 stars but none at 4 or 5. CMFs are for corridor level. CMFs for corridor and site level. Regulate median opening spacing 2 CMFs at 3 stars for site level. CMFs for corridor and site level. Manage location and spacing of unsignalized access Establish spacing for unsignalized access CMFs at 3 stars for site and corridor levels exist. CMFs for corridor, intersection and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) NCHRP project 17-62 (includes data from Ohio, Minnesota, and North Carolina) Iowa State Project: Development of a Comprehensive Access Management Policy and Guidelines Brigham Young University Project: Safety Analysis of Access Management Implementation in Utah Install non- traversable medians, and accommodate left- turns and U-turns Convert traversable median (non-TWLTL) to non-traversable median 2 CMFs at 3 stars exist for intersection level. 2 CMFs at 4 stars exist for corridor and site level. CMFs for intersection level and improved CMFs for corridor and site level and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) NCHRP project 17-62 (includes data from Ohio, Minnesota, and North Carolina) Brigham Young University Project: Safety Analysis of Access Management Implementation in Utah

68 Strategy Sub-strategy CMF Availability Missing CMFs Possible Data Sources Replace TWLTL with non-traversable median CMFs rated 3 stars exist at site and corridor level. Higher rated CMFs for corridor, intersection and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) NCHRP project 17-62 (includes data from Ohio, Minnesota, and North Carolina) Brigham Young University Project: Safety Analysis of Access Management Implementation in Utah Manage the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges Establish spacing criteria for interchange ramp terminals 3 CMFs rated 3 stars available at site level. CMFs for corridor, intersection and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina)

69 Table 50. Gaps NOT identified as high priority. Strategy Sub-strategy CMF Availability Missing CMFs Possible Data Sources Convert two-way to one-way Convert two-way operation to one-way operation 8 CMFs rated 3 stars for site level. CMFs for corridor, intersection and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) NCHRP 17-58 “Safety Prediction Models for Six-Lane and One-Lane Urban Arterials” Establish corner clearance criteria Driveways at signalized intersections 15 CMFs rated 3 stars for intersection level. CMFs for corridor and intersection level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) Safety Effects of Corner Clearance at Signalized Intersections. FHWA DCMF Project (includes data from California and North Carolina). NCHRP project 17-62 (includes data from Ohio, Minnesota, and North Carolina) Install traversable medians Convert undivided to divided by traversable median 1 CMF rated 3 for both site and corridor level. CMFs for corridor, intersection and site level. Manage spacing of traffic signals Establish traffic signal density criteria 3 CMFs rated 3 for both site and corridor level. CMFs for corridor and site level. DTFH61-09-C-00026 Safety Evaluation of Access Management Policies and Techniques (includes data from California, Minnesota, and North Carolina) NCHRP project 17-62 (includes data from Ohio, Minnesota, and North Carolina) Establish traffic signal spacing criteria 7 CMFs rated 3 for both site and corridor level. CMFs for corridor and site level. Right-turn treatment Channelize right-turn lane 3 CMFs rated 1 star for intersection level. CMFs for intersection and site level.

70 Prioritization of NCHRP 17-74 Research The previous section presents a summary of current knowledge on the safety effects of access management strategies as well as knowledge gaps and potential priority research topics. In a discussion of these results with the NCHRP 17-74 panel, members indicated a preference to focus on enhancing the current Highway Safety Manual (1st Edition) Part C Predictive Method to include access management features. This included the calibration of existing SPFs to different access management conditions, developing adjustment factors for existing SPFs to capture the effects of access management strategies, and developing CMFs for specific access management strategies. This section presents opportunities to fill primary gaps in CMFs and SPFs related to access management. Specifically, this section identifies opportunities to develop CMFs that are compatible with, and complementary to the Part C Predictive Method in the Highway Safety Manual (1st Edition), including: 1. A summary of which access management features are directly or indirectly addressed in the Highway Safety Manual (1st Edition) Predictive Method. 2. A summary of the priority access management features that are candidates for CMF development in this project (based on discussion between the project team and Panel). Summary of Access Management Features Directly or Indirectly Addressed in the Highway Safety Manual (1st Edition) The Part C Predictive Method in the Highway Safety Manual (1st Edition) was reviewed to identify which access management features are addressed directly or indirectly through SPFs and CMFs. The list of access management features is the same list used to categorize these features for the literature review and gap analysis. Table 51 lists the access management features and indicates which are accounted for in the Part C Predictive Method. A check mark indicates that the Highway Safety Manual (1st Edition) provides a CMF for that strategy. “Indirect – Use of Multiple SPFs Possible” means that the Highway Safety Manual (1st Edition) has SPFs for the alternate site characteristics. They may be used to infer a CMF provided that each of the SPFs can first be successfully calibrated to the same jurisdiction of interest and the sites to which the CMF would apply are indeed represented by the SPFs in question in terms of site characteristics and traffic volumes. The shaded cells indicate the highly-ranked access management strategies based on the survey of practitioners. Priorities included median type, corner clearance, access density, access spacing, turn lanes, and turn restrictions. There are limited features currently accounted for in the SPFs and CMFs within the Highway Safety Manual (1st Edition), and even fewer that are explicitly provided. While analysts can infer the effect of some strategies from SPFs, caution should be exercised in doing so because differences in crash predictions from two SPFs representing two levels of a feature could be due to factors other than the feature of interest that are not accounted for in the SPFs.

71 Table 51. Summary of access management strategies in Highway Safety Manual (1st Edition) ( indicates a CMF is available, -- indicates no CMF is available). Strategy Substrategy Chapter 10 (Rural 2 lane) Chapter 11 (Rural Multi-lane) Chapter 12 (Urban/Suburban arterials) Chapter 18 Freeways Chapter 19 Ramps Alternative intersection and interchange design Convert from 4-legged to two 3-legged intersections Indirect – Use of Multiple SPFs Possible No SPF for 3-leg signal Indirect – Use of Multiple SPFs Possible No SPF for 3-leg signal Indirect – Use of Multiple SPFs Possible -- Install roundabout at roadway intersection Indirect – Use of Multiple SPFs Possible Updates from NCHRP 17-70 has SPFs Indirect – Use of Multiple SPFs Possible Updates from NCHRP 17-70 has SPFs Indirect – Use of Multiple SPFs Possible Updates from NCHRP 17-70 has SPFs Indirect – Use of Multiple SPFs Possible Updates from NCHRP 17-70 has SPFs Provide median acceleration lane -- -- -- -- Replace direct left-turn with grade-separated interchange Indirect – Use of Multiple SPFs Possible SPFs for interchange elements available in new freeway chapter material Indirect – Use of Multiple SPFs Possible SPFs for interchange elements available in new freeway chapter material Indirect – Use of Multiple SPFs Possible SPFs for interchange elements available in new freeway chapter material Indirect – Use of Multiple SPFs Possible NCHRP 17-45 has SPFs Superstreet (RCUT, J-Turn) -- -- -- -- Jug handle intersections -- -- -- -- Install continuous green T intersection at signalized 3- legged intersection -- -- -- -- Control driveway design elements Change class/type of driveway -- -- Indirect – Use of Multiple SPFs Possible SPFs in current Highway Safety Manual (1st Edition). NCHRP 17-62 recommends driveway density though --

72 Strategy Substrategy Chapter 10 (Rural 2 lane) Chapter 11 (Rural Multi-lane) Chapter 12 (Urban/Suburban arterials) Chapter 18 Freeways Chapter 19 Ramps Change movement restriction -- -- -- -- Require design of driveways with the appropriate return radii, throat width, channelization, number of lanes and throat length for the type of traffic to be served -- -- -- -- Control intersection design elements Control turning radius -- -- -- -- Convert two-way to one- way Convert two-way operation to one-way operation -- -- Indirect – Use of Multiple SPFs Possible SPFs may be available from NCHRP 17-58 -- Establish corner clearance criteria Driveways at signalized intersections -- -- -- -- Improve cross- connectivity Allow vehicles to access adjacent properties without returning to the mainline -- -- -- -- Install continuous TWLTL on undivided highway Non-road diet scenarios Direct - CMF or SPF adjustment factor available -- Indirect – Use of Multiple SPFs Possible SPFs available -- Road diet scenarios -- --  Indirect – Use of Multiple SPFs Possible SPFs available -- Install non-traversable medians, and accommodate left-turns and U-turns Convert traversable median (non-TWLTL) to non- traversable median -- -- -- -- Install isolated median barriers -- -- -- -- Install non-traversable median on undivided highway -- Indirect – Use of Multiple SPFs Possible SPFs available Indirect – Use of Multiple SPFs Possible SPFs available --

73 Strategy Substrategy Chapter 10 (Rural 2 lane) Chapter 11 (Rural Multi-lane) Chapter 12 (Urban/Suburban arterials) Chapter 18 Freeways Chapter 19 Ramps Replace TWLTL with non- traversable median -- -- Indirect – Use of Multiple SPFs Possible SPFs available -- Install service or frontage roads Change proportion of primary roadway with frontage road -- -- -- -- Install frontage road to provide access to individual parcels -- -- -- -- Install traversable medians Convert undivided to divided by traversable median -- -- -- -- Left-turn treatment Change storage capacity of existing left-turn deceleration lane -- -- -- -- Channelize left-turn lane -- -- -- -- Control/improve design elements of left-turn lanes -- -- -- -- Install left-turn deceleration lanes at roadway intersections -- -- -- -- Prohibit left turn -- -- -- -- Manage location and spacing of unsignalized access Establish density for unsignalized access (e.g., maximum driveway density) Direct - CMF or SPF adjustment factor available -- Indirect – Use of Multiple SPFs Possible SPFs available in NCHRP 17-62 report -- Establish spacing for unsignalized access (e.g., driveways) -- -- -- -- Manage spacing of traffic signals Establish traffic signal density criteria -- -- -- -- Establish traffic signal spacing criteria -- -- -- -- Manage the location, spacing, and design of Create directional median opening -- -- -- --

74 Strategy Substrategy Chapter 10 (Rural 2 lane) Chapter 11 (Rural Multi-lane) Chapter 12 (Urban/Suburban arterials) Chapter 18 Freeways Chapter 19 Ramps median openings and crossovers Install U-turns as an alternative to direct left turns -- -- -- -- Regulate median opening density -- -- -- -- Regulate median opening spacing -- -- -- -- Replace full median opening with median designed for left turns from the major roadway -- -- -- -- Manage the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges Establish spacing criteria for interchange ramp terminals -- -- -- -- Provide adequate sight distance at access points Manage design elements to improve sight distance -- -- -- -- Manage the location and placement of parking (e.g., replace curb parking with off- street parking or restrict on- street parking near driveways or intersections to improve sight distance) -- -- -- -- Manage vegetation to improve sight distance (e.g., in landscaped medians or sight triangles) -- -- -- -- Right-turn treatment Channelize right-turn lane -- -- -- -- Control/improve design elements of right-turn lanes -- -- -- -- Install right-turn deceleration lane at roadway intersections -- -- -- --

75 CMFs that are directly or indirectly accounted for in the Highway Safety Manual (1st Edition) are briefly described below. Illustrative examples of inferring CMFs are provided for two features: 1) install non- traversable median on undivided highway, and 2) establish density for unsignalized access (e.g., maximum driveway density). In both cases, there are CMFs provided Part D. These can be compared to CMFs that could be inferred from the SPFs provided in Part C. Convert from 4-legged to two 3-legged intersections The effect on safety from converting from a single 4-legged intersection to two 3-legged intersections can be inferred from SPFs. Chapters 10, 11, and 12 have SPFs for 4-legged and 3-legged intersections for both signalized and unsignalized control. The exception is for 3-legged signalized intersections in chapters 10 and 11, which do not provide those SPFs. Install roundabout at roadway intersection The effect on safety from converting to a roundabout can be inferred from the SPFs available. Roundabout SPFs have been developed from NCHRP Project 17-70 and will be included in the Highway Safety Manual (2nd Edition). As mentioned above, SPFs are available for almost all intersection types that may exist prior to roundabout conversion. Replace direct left-turn with grade-separated interchange The effect on safety from converting to an interchange can be inferred from the SPFs available. SPFs for freeways and interchanges have been developed from NCHRP Project 17-45 and will be included in the Highway Safety Manual (2nd Edition). As mentioned above, SPFs are available for almost all intersection types that may exist prior to roundabout conversion. Change class/type of driveway Chapter 12 of the Highway Safety Manual (1st Edition) predicts crashes by type of driveway. CMFs for changing the class/type of driveway can be inferred from these SPFs. Convert two-way operation to one-way operation NCHRP project 17-58 is developing SPFs for one-way roads that could be used with SPFs from chapter 12 of the Highway Safety Manual (1st Edition) to infer a CMF for converting to one-way operation. Install continuous TWLTL on undivided highway Chapter 10 includes a CMF for this treatment. For chapter 12, there are SPFs for two-lane roads and four- lane roads as well as SPFs for three or five-lane including a TWLTL. A CMF can be inferred by comparing the predictions if a TWLTL is to be added without taking away through traffic lanes. Install non-traversable median on undivided highway SPFs in chapters 11 and 12 can be used to infer a CMF. For example, chapter 12 provides SPFs for multi- vehicle driveway and non-driveway crashes per mile, considering the AADT, the number and type of driveways, and whether the arterial has a median or not. Table 52 presents inferred CMFs for providing a median on urban and suburban arterials, where the CMFs depend on AADT, driveway density, and

76 driveway type. In principle, these CMFs may be preferred to CMFs that are explicitly provided in Part D that do not account for changes in AADT, driveway density, and driveway type. Table 52. CMFs for providing median based on Highway Safety Manual (1st Edition) Part C Predictive Method for urban four-lane undivided (4U) and divided (4D) arterials. AADT Base model (no driveways) Major Commercial Major Residential 40/mile 20/mile 40/mile 20/mile 5000 0.635 0.456 0.635 0.623 0.847 10000 0.648 0.471 0.657 0.645 0.871 15000 0.656 0.481 0.671 0.659 0.884 20000 0.662 0.488 0.681 0.668 0.894 25000 0.666 0.494 0.689 0.676 0.901 30000 0.670 0.499 0.695 0.682 0.907 35000 0.673 0.503 0.701 0.687 0.911 40000 0.676 0.506 0.705 0.692 0.915 Replace TWLTL with non-traversable median SPFs in chapter 12 can be used to infer a CMF by comparing the prediction for five-lane including a TWLTL to the prediction for a four-lane divided road. Establish density for unsignalized access (e.g., maximum driveway density) Chapter 10 provides a CMF and chapter 12 provides SPFs that can be used to infer a CMF. For example, Table 53 presents CMFs inferred from the chapter 12 SPFs. As before, SPFs are provided for multi-vehicle driveway and non-driveway crashes per mile, considering the AADT, the number and type of driveways, and whether or not the arterial is divided. Thus, the inferred CMFs for changing driveway density also depend on these factors. In principle, these CMFs are preferred to the Part D CMFs, at least within the range of the data used to estimate the SPFs from which they are inferred because the impact on crashes is disaggregated by AADT, the number and type of driveways, and whether the arterial is divided. Table 53. Driveway density CMFs inferred from the Highway Safety Manual (1st Edition) Part C Predictive Method for multi-vehicle crashes on urban four-lane undivided (4U) and divided (4D) arterials. AADT Reduce Driveways from 40 to 30/mile Reduce Driveways from 20 to 10/mile Major commercial Major residential Major commercial Major residential 4U 4D 4U 4D 4U 4D 4U 4D 5000 0.817 0.886 0.853 0.922 0.712 0.895 0.791 0.937 10000 0.823 0.922 0.859 0.950 0.725 0.908 0.804 0.945 15000 0.826 0.927 0.863 0.954 0.733 0.914 0.812 0.949 20000 0.829 0.930 0.866 0.956 0.739 0.919 0.817 0.952 25000 0.830 0.933 0.868 0.958 0.743 0.922 0.821 0.954 30000 0.832 0.935 0.870 0.959 0.747 0.925 0.825 0.956 35000 0.833 0.936 0.872 0.961 0.750 0.927 0.827 0.957 40000 0.835 0.938 0.873 0.962 0.753 0.929 0.830 0.958

77 Summary of Priority Access Management Features with Potential for CMF Development Considering the literature review, the survey of practitioners, and discussion with the project panel, Table 54 identifies the priority access management features for this research project. For these strategies, there were also no CMFs rated 4 or 5 stars in the CMF Clearinghouse at the time of the research project. Table 54. High-priority strategies. Strategy Substrategy CMF Availability Manage location, spacing, and design of median openings and crossovers Create directional median opening No CMFs rated 3 or more stars Regulate median opening density 5 CMFs rated 3 stars. CMFs are for corridor level. Regulate median opening spacing 2 CMFs rated 3 stars for site level. Manage location and spacing of unsignalized access Establish spacing for unsignalized access 13 CMFs rated 3 stars. CMFs are for corridor and site level. Manage the spacing of signalized and unsignalized access on crossroads in the vicinity of freeway interchanges Establish spacing criteria for interchange ramp terminals 3 CMFs rated 3 stars available at site level. Establish corner clearance criteria Driveways at signalized and unsignalized intersections 15 CMFs rated 3 stars for intersection level. Manage spacing of traffic signals Establish traffic signal density criteria 3 CMFs rated 3 stars for both site and corridor level. Establish traffic signal spacing criteria 7 CMFs rated 3 stars for both site and corridor level. Right-turn treatment Channelize right-turn lane 3 CMFs rated 1 star for intersection level.

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The 1st Edition, in 2010, of the AASHTO Highway Safety Manual revolutionized the transportation engineering practice by providing crash modification factors and functions, along with methods that use safety performance functions for estimating the number of crashes within a corridor, subsequent to implementing safety countermeasures.

The TRB National Cooperative Highway Research Program's pre-publication draft ofNCHRP Research Report 974: Application of Crash Modification Factors for Access Management, Volume 2: Research Overview documents the research process related to access management features.

Supplementary to the report is the pre-publication draft of NCHRP Research Report 974: Application of Crash Modification Factors for Access Management, Volume 1: Practitioner’s Guide and a summary presentation for the two volumes.

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