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NCHRP 17-59 1 SUMMARY Most multiple vehicle crashes at three- and four-leg intersections with stop control on the minor road can be classified as either gap acceptance crashes or traffic control device violations (e.g., stop sign violations), with gap acceptance crashes generally occurring more frequently. Gap acceptance crashes include several crossing path crash types: right turn into path, left turn into path, left turn across path, and straight crossing path. Intersection sight distance (ISD) is a key design element at intersections. It contributes to the ability of drivers on the minor road to identify an appropriate gap for departing from the intersection and entering or crossing the major road. Designs that provide adequate ISD also allow drivers on the major road approach to see stopped vehicles on the minor road approach so that they can be more aware and prepare to slow or stop if needed. A Policy on Geometric Design of Highways and Streets, 6th Edition (the Green Book), published by the American Association of State Highway and Transportation Officialsâ (AASHTO), provides design criteria for minimum sight distances, including ISD. The ISD criteria in the Green Book vary according to the minor road traffic control, design speed of the major road, and turning movement from the minor road. The minimum ISD values are based on driver gap acceptance behavior. Assumptions are made on physical conditions (e.g., object height and driver eye height), vehicle performance capabilities, and driver behavior. Designing an intersection with the minimum required ISD or above does not necessarily mean that crashes related to gap acceptance behavior will not occur. Available ISD may change over time (e.g., through development of adjacent land parcels, overgrown vegetation, seasonal crop growth). Additional sight distance may also be needed in certain situations. Finally, adequate ISD alone is not all a driver needs to identify and use a suitable gap. An understanding of the relationship between ISD and safety at stop-controlled intersections is needed, with potential applications of this knowledge to both performance-based design and substantive road safety management practices. The overall objectives of this research were to: 1) estimate the relationship between available ISD and safety and, 2) develop guidelines for transportation agencies to use when making decisions about ISD. The first phase of the project was an extensive review of ISD-related literature and documented state and local practices related to defining ISD parameters and making ISD measurements in the field. The literature review revealed that ISD-related safety studies are inconsistent in both the direction and magnitude of the major factors that impact both crash frequency and ISD measurements at intersections. The results of both the literature and practice reviews ultimately guided the study design and overall research direction, including selecting the field measurement parameters and methodologies. Data were gathered from a total of 832 intersections in North Carolina, Ohio, and Washington. Variables characterizing crash experience, roadway features, and traffic/operations were
NCHRP 17-59 2 collected using Geographic Information Systems (GIS) databases, Highway Safety Information System (HSIS) data files, and field measurements. The data were then analyzed using multivariable count regression models. The safety effects of available ISD differed by two-way annual average daily traffic (AADT) on the major roadway as well as by speed limit on the major road; therefore, main effects and interaction terms were included in final model specifications. 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 crash frequency in the database represents the crash frequency for one minor road and major road approach combination). Estimation results for two multivariable count regression models are reported: ï· Total (i.e., all severities) target crashes; and ï· 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. Results suggest 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 and minor road traffic volumes and the major road speed limit. The sensitivity of the expected number of crashes to changes in ISD increases as both traffic volumes and speed limits increase. Crash modification functions (CMFunctions) for each of the target crash types were estimated using the regression models. Applicability of the CMFunctions varies by available ISD, major road AADT, and speed limits available in the dataset. The final step in the study was to translate the research results into a series of charts for practitioners to efficiently reference when conducting an intersection safety analysis focused on ISD. The following charts are provided: ï· ISD CMFunctions for the expected number of total (i.e., all severities) target crashes at minor road, stop-controlled intersections with major road speed limits of 35, 40, 45, 50, 55, and 60 mph; and ï· ISD CMFunctions for the expected number of target fatal and injury crashes at minor road, stop-controlled intersections with major road speed limits of 35, 40, 45, 50, 55, and 60 mph.
NCHRP 17-59 3 The charts are included in a stand-alone guidance document. The guidance document also includes a summary of key background information on ISD and step-by-step instructions on how and when to use the ISD CMFunction charts. Additional information on low-cost countermeasures and other ISD-related resources are also provided for practitioners to assist them in identifying other measures that may be applicable for making intersection safety improvements.