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1 Guidance for Evaluating the Safety Impacts of Intersection Sight Distance 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 on Highways and Streets, 6th Edition, also known as the Green Book (AASHTO 2011), provides design criteria for minimum sight distances, includ- ing ISD. The ISD criteria in the Green Book vary according to the minor-road traffic con- trol, 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 capa- bilities, 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 par- cels, overgrown vegetation, or 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. The overall objectives of this research were to (1) estimate the relationship between avail- able ISD and safety and (2) develop guidelines for transportation agencies to use when making decisions about ISD. Data were gathered from a total of 832 intersections in North Carolina, Ohio, and Wash- ington. Variables characterizing crash experience, roadway features, and traffic/operations were collected using geographic information system (GIS) databases, Highway Safety Infor- mation 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 roadway; 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 fre- quency in the database represents the crash frequency for one minor-road and major-road approach combination). S U M M A R Y
2 Guidance for Evaluating the Safety Impacts of Intersection Sight Distance 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. Results suggest that the expected number of target crashes is associated with avail- able ISD. Target crash frequencies increase as available ISD decreases. The results of this research also suggest that ISD is associated with expected crash frequency in a nonlinear fashion. The sensitivity of the expected number of target crashes to changes in ISD is high- est when ISD is shorter, and decreases as ISD increases (i.e., the safety benefit of increasing ISD from 300 to 600 ft is substantially larger than the safety benefit of increasing ISD from 1,000 to 1,300 ft). 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 traf- fic 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 avail- able 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: ⢠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 ⢠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. The charts are included in this guidance document, which 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 is also provided for practitioners to assist them in identi- fying other measures that may be applicable for making intersection safety improvements.
