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196 C H A P T E R 1 1 Conclusions The objectives of this project were to develop and refine CMFs and SPFs for access management features and to develop guidance to assist transportation agencies in quantifying the safety impacts of their decisions related to access management. Specifically, this project: ï· Verified the reliability of using existing SPFs from the Highway Safety Manual (1st Edition) to quantify the safety performance of urban and suburban arterials, ï· Quantified the safety performance of access management features by developing and refining CMFs and SPFs for various levels of analysis (site, segment/intersection, and corridor), ï· Provided guidance for the use of access management related CMFs from the Highway Safety Manual (1st Edition) and the CMF Clearinghouse, and ï· Identified opportunities for future research. In developing the guidance, this project: ï· Recommended and prioritized CMFs for individual access management strategies at various levels (site, segment/intersection, and corridor) for various categories, ï· Assessed and recommended methods to quantify the cumulative and interactive effects of access management features, and ï· Recommended procedures to aid with evaluating and documenting access management planning and design decisions. The objectives were accomplished by assessing the reliability of the Part C Predictive Method from the Highway Safety Manual (1st Edition) and developing CMFs (factors or functions) for the priority access management features that can be applied with the existing Part C Predictive Method. While this research focused on chapter 12 of the Highway Safety Manual (1st Edition) for urban and suburban arterials, it also considered the SPFs developed for NCHRP Project 17-62 for crashes by type and severity. For the 17-62 SPFs, no CMFs are available, so the exercise only applied the SPFs using data that represent the base conditions. For segment-level predictions, the Part C Predictive Method accounts for the number and type of driveways along the segment. For intersection-level predictions, the Part C Predictive Method accounts for the presence of left- and right-turn lanes, left-turn signal phasing (at signalized intersections), and right- turn-on-red restrictions (at signalized intersections). The research from NCHRP Project 17-74 confirmed the Highway Safety Manual (1st Edition) Part C Predictive Method performs relatively well across a range of several other access management features not accounted for in the existing predictive method. Specifically, the Part C Predictive Method (i.e., combination of SPFs and CMFs) performs well for sites with similar geometry, but different access management features such as median opening spacing, number of median openings by type, and corner clearance along a segment. There are, however, a few scenarios where the existing models do not perform well across sites with different access management features. Notable scenarios include locations with channelized right-turn lanes and locations with nearby ramp terminals. As such, this research developed adjustment factors to account for differences in the predictions at locations with channelized right-turn lanes and locations with nearby ramp terminals. The following is a brief summary of those results.
197 ï· For distance to ramp terminal, bias was not observed in the predictions for signalized intersections. The CURE plots did not show substantial bias for stop-controlled intersections although the CURE plot does indicate that for distances under 1,500 feet, there may be an underprediction of crashes. The recommended CMF (adjustment factor) for three- and four-legged stop-controlled intersections is 2.12 if there is a ramp terminal within 1,500 feet. The CMF is intuitive in that more crashes are predicted as the distance to ramp terminal is smaller. ï· For channelizing right-turn lanes, the results were inconsistent and generally not statistically significant at the 95-percent confidence level for three- and four-legged signalized intersections and four-legged stop-controlled intersections. For three-legged stop-controlled intersections, the recommended CMF (adjustment factor) is 0.72 for the presence of a channelized right-turn lane, compared to the base condition of a right-turn lane with no channelization. Another significant finding of this research is that the Part C Predictive Method should not be used to estimate the safety effect of variables related to access spacing and density. The Part C Predictive Method is only applicable to estimating the safety performance of individual segments and intersections, assuming independence among each unit of analysis. While the results can be aggregated from multiple segments and intersections to estimate the safety performance of a corridor, as suggested in the Highway Safety Manual (1st Edition), this method does not consider the potential interactions among adjacent or nearby sites (e.g., access spacing and density). The existing Part C Predictive Method may even produce counterintuitive results (e.g., fewer estimated segment crashes with an increase in the number of intersections along a corridor). The structure of the data for the Highway Safety Manual (1st Edition) Part C Predictive Method is such that roadways are segmented by intersection and midblock areas and crashes are assigned to one of the two. One of the important findings of this research found that as the number of intersections in a segment increased, the expected number of segment crashes often decreased. This is contrary to expectations since the presence of more intersections should lead to more conflicts, and certainly not fewer, even away from the intersections. However, the structure of the data may contribute to this counterintuitive finding. Specifically, if more intersections are present, then it is more likely that a crash will be assigned to an intersection and not a segment. Given this finding, the data are not conducive to segment-level analysis of some variables (e.g., intersection density, spacing, etc.). Instead, these variables are better assessed at the corridor level. The corridor-level predictive method presented in the Practitioner Guide is more appropriate for considering interactions among access management features and estimating the safety effect of variables related to access spacing and density. Future Research In conducting this research, the project team identified several potential research opportunities. First, there are still gaps in quality CMFs as identified in chapter 4. While this research helped to assess and improve the reliability of the Part C Predictive Method, including the exploration and development of new CMFs and adjustment factors, there are opportunities to enhance CMFs for strategies such as the following: ï· Provide median acceleration lane (Alternative Intersection and Interchange Design), ï· Jug handle intersections (Alternative Intersection and Interchange Design), ï· Install frontage road to provide access to individual parcels (Install Service or Frontage Roads), ï· Create directional median opening (Manage the Location, Spacing, and Design of Median Openings and Crossovers), ï· Manage design elements to improve sight distance (Provide Adequate Sight Distance at Access Points), ï· Manage the location and placement of parking (Provide Adequate Sight Distance at Access Points), and ï· Channelize right-turn lane (Right-Turn Treatment), and ï· Create access to adjacent properties without returning to the mainline (Improve Cross-Connectivity).
198 The following are opportunities to improve SPFs: ï· Develop SPFs that account for median acceleration lanes, superstreet, or jug handle designs, ï· Develop SPFs that account for changes in driveway movement restrictions or design parameters (type of driveway, radii, throat width, channelization, number of lanes, or throat length), ï· Develop SPFs that account for installation of service or frontage roads, ï· Develop SPFs that account for installation of traversable medians, ï· Develop SPFs that account for intersections with different left-turn treatments (length of left-turn deceleration lanes, channelized left-turn lanes, design elements of left-turn lanes, presence of left-turn deceleration lanes, provision of turning by-pass lanes, or prohibition of left turns), ï· Develop SPFs that account for directional median openings, U-turns as an alternative to direct left-turns, median opening spacing, and median openings designed for left-turns from the major roadway, and ï· Develop SPFs that account for sight distance at access points. In exploring potential new CMFs, the project team also identified counterintuitive results and inconsistencies in some of the results from state to state. The following are opportunities to further explore counterintuitive results: ï· CMFs were counterintuitive for limited corner clearance on the upstream corner of an intersection, indicating statistically significant decreases in total, fatal and injury, and rear rear-end crashes, ï· CMFs were counterintuitive for distance to ramp terminal at signalized intersections, indicating fewer crashes when ramp terminals are within 1,500 feet of the intersection, and ï· CMFs were counterintuitive for variables related to access density and spacing along a segment, indicating fewer segment crashes with an increase in the number of intersections along a corridor. Finally, the focus group suggested an opportunity to consider conflict rate analysis in future research. If a method could be developed to relate the number of conflicts, potentially by movement, to the safety performance of different facilities and access management features, then this could provide a useful tool for practitioners. This is related to the Safe System Approach, which seeks to minimize the number of conflict points, severity of potential impact angles, and speed of impact.