Safety Impacts of Intersection Sight Distance (2018)

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NCHRP 17-59 87 CHAPTER 4: CONCLUSIONS, RECOMMENDATIONS, AND SUGGESTED RESEARCH CONCLUSIONS AND RECOMMENDATIONS This study included 832 minor road stop-controlled intersection approaches, which were approximately evenly selected across North Carolina, Ohio, and Washington. The selected sites represent a wide variety of area types, geometric conditions, and traffic volumes and were selected independent of crash history. A variety of roadway, roadside, traffic, and safety data were collected at each site. Field site visits were performed to measure available ISD and other characteristics of the roadway and roadside. Traffic and crash data were collected from the HSIS database for the most recently available four-year period. Minor road approaches without traffic volume data were included by obtaining estimates through two-hour counts. A series of analytical models were developed based on the data to explore the relationship between crashes, available ISD, and other factors. The field studies also allowed for development of a standardized methodology for measuring ISD. The field measurements included identifying the location of the driver selected decision point, which matched the AASHTO Green Book recommended location 83 percent of the time, confirming that this location reflects the drivers’ behavior. A cross-sectional study design with count regression modeling was used to quantify the relationship between safety and ISD. Target crashes were defined as those where a vehicle on the minor road collided with a vehicle on the major road. Target crash counts were associated with specific, measured values of an approach level ISD (i.e., one observation of target crash frequency in the database represents the target crash frequency for one minor road and major road approach combination). Estimation results for two multivariable count regression models are reported:  Total (i.e., all severities) target crashes; and  Target fatal and injury crashes. Models for target fatal and incapacitating injury crashes, target angle crashes, and target daytime crashes were also explored and uncovered similar trends between the frequency of these crash types and ISD. In some cases, the statistical significance of estimated model parameters decreased due to the smaller numbers of these more refined target crash type definitions. The analyses indicated that the expected number of target crashes is associated with available ISD. Target crash frequencies increase as available ISD decreases. Results of this research also suggest that ISD is associated with expected crash frequency in a non-linear fashion. The sensitivity of the expected number of target crashes to changes in ISD is highest when ISD is shorter, and decreases as ISD increases (i.e., the safety benefit of increasing ISD from 300 to 600 feet is substantially larger than the safety benefit of increasing ISD from 1,000 to 1,300 feet). The results also suggest that the impacts of ISD on crash frequencies vary as a function of the major road two-way AADT and the major road speed limit. The sensitivity of the expected

NCHRP 17-59 88 number of crashes to changes in ISD increases as both traffic volume and speed limit increase. CMFunctions for each of the target crash types were estimated using the regression models. Applicability of the CMFunctions varies by available ISD, major road AADTs, and speed limits available in the dataset. An exploratory measure of ISD quality was developed and was found to be associated with target crash and target fatal and injury crash frequency. Approaches with more objects on the roadside were found to be associated with fewer crashes, which suggests that these objects may aid minor stop-controlled drivers in estimating the speed of vehicles approaching on the major roadway and improve their subsequent gap selection. However, this result should be viewed with caution, as large objects positioned close to the roadway may restrict sight distance and could increase the injury risk for single vehicle run-off-road crashes. CMFunctions were developed as part of this research, relating major road AADT and available ISD to expected crash occurrence. To aid in practitioner implementation of these results, the CMFunctions were translated into a graphical format, which allowed for estimation of the safety impacts associated with changes in available ISD over the applicable range of traffic volumes and speed limits. The guidance developed as part of this research can be used by practitioners to estimate the safety impacts of changes to available ISD. In addition to ISD improvements, practitioners should also consider other low-cost countermeasures that may be implemented that effectively reduce crashes at these types of intersections. The guidance includes information on resources that can be used to identify other measures that may be applicable. INCORPORATING RESULTS INTO PRACTICE The results developed here are presented in a stand-alone guidance document that practitioners can use to assess the safety impacts of intersection sight distance at intersections with minor road stop control. Additionally, these results can be implemented into field data collection procedures manuals, safety manuals, and design manuals. This research developed a low-cost, efficient, and repeatable method for collecting available ISD in the field. The method produces reliable results using a two-person data collection team and minimal equipment. This method could be integrated into field data collection procedures manuals such as the ITE Traffic Engineering Studies Handbook or state and local handbooks for collecting ISD. The primary finding of this research is the safety relationship between ISD and crashes. This relationship is suitable to be included in the next edition of the Highway Safety Manual. Specifically, the results can readily be incorporated into Chapter 14, Part D Crash Modification Factors, of the HSM. The results will be presented to the Transportation Research Board (TRB) Highway Safety Performance Committee as part of the research results dissemination. The graphs from the guidance document could be included or, if the Committee’s preference is for more succinct content, the CMFs for unknown AADT could be explicitly included in the HSM

NCHRP 17-59 89 to the guidance document for the full results when the volume is known. The description of the CMF in the guidance must emphasize that the CMF is applicable for an approach direction calculation and applies to rural, two-lane highways; rural, multilane highways; and urban streets. The methodology for developing an intersection level CMF from the approach level CMFs can be included. Additionally, the CMF is applicable for direct changes to values of available ISD and utilizes the existing condition as the base condition for the analysis. Design manuals such as the AASHTO Green Book should also incorporate the results of this research. Manuals such as this would not include the detailed results but instead the general finding that greater ISD is associated with fewer target and target fatal and injury multivehicle crashes at intersections. The findings of this research do not change the values for design ISD presented in the Green Book, but instead provide further evidence of the relationship between available ISD and crashes. Furthermore, reference to the guidance document within the Green Book would assist practitioners with both implementation of the design ISD guidelines and method for measuring available ISD. DISSEMINATION For the guidance document to be implemented, the research results must be broadly disseminated. The project team initiated dissemination efforts by developing a dissemination plan that identifies the primary audiences and key opportunities and strategies for reaching broad audiences. Key opportunities for dissemination include the following:  Industry conferences and professional organizations. Examples include the TRB Annual Meeting, AASHTO Standing Committee meetings, and the Institute of Transportation Engineers (ITE) Annual meeting.  Professional organization websites and email databases, such as AASHTO, American Society of Civil Engineers (ASCE), National Society of Professional Engineers, and the Federal Highway Administration (FHWA).  Industry journals and publications including the FHWA Innovator, the AASHTO Journal, the ITE Journal, Transportation Research Record, and Governors Highway Safety Association.  Transportation-related websites such as the TRB National Cooperative Highway Research Program Reports website, the National Highway Traffic Safety Administration, the Crash Modification Factors Clearinghouse, and Traffic Safety Marketing. To further assist with dissemination, the project team developed a PowerPoint Presentation slide deck that can be easily adapted to fit varying audiences and presentation lengths. It is available through the NCHRP Senior Program Officer.

NCHRP 17-59 90 SUGGESTED FUTURE RESEARCH The following topics were identified for future ISD research:  Future research should consider the difference in crash modification factors for intersections that have higher ISD-related crash counts. It is likely that intersections with histories of higher ISD-related crash counts will benefit further from changes to available ISD. This would likely include oversampling at locations where limited ISD exists.  Additional exploration of the effects of dynamic ISD on safety would be beneficial. This would include not only seasonal changes, but also effects of on-street parking and other non-static objects that affect ISD.  Future research should consider the impact of right turn lanes on the major roads. It was unclear from this research if the increase in crash frequency found for sites with right turn lanes on the major road was due to site selection bias or due to the physical presence of the right turn lanes.  Since this research focused on crashes at the approach direction level, crashes were identified and attributed to a specific departure triangle. This limited the study to only crashes involving a vehicle on the major road and a vehicle from the minor road. Available ISD may be related to other crash types that do not fall within this parameter (e.g., rear-end crashes). Future research should examine the relationship between available ISD and total crashes at an intersection. This analysis can be completed at the intersection level and may differentiate results by facility and intersection type.  Further exploration of the safety effects of roadside visual clutter (or ISD “quality”) should be performed. This may include a human factors study investigating the effects of nearside and farside objects on the estimation of speed and gap size of vehicles traveling on the major roadway.

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