The provision of appropriate intersection sight distance (ISD) is an important element in intersection design. The approach to the determination of ISD in AASHTO’s A Policy of Geometric Design of Highways and Streets (known as the “Green Book”) is based on gap-acceptance developed in the 1996 NCHRP Report 383: Intersection Sight Distance. This approach includes the ability to more easily calculate ISD for both passenger cars and trucks by allowing the selection of an ISD “design vehicle.” Calculations of ISD using this approach yield different results than those calculated with earlier methods. However, past research efforts to analyze and quantify the safety impacts of ISD have produced inconsistent results, making it difficult to fully evaluate the different approaches. The quantification of these safety impacts would allow better design evaluations that include variations in available ISD. It would also provide the opportunity to evaluate the potential safety impacts of the ISD criteria in the Green Book. Transportation agencies would benefit from guidance on incorporating the safety impacts of intersection sight distance into the decision-making process.
The objective of this research is to determine the relationships between safety and available ISD and to develop guidelines for transportation agency decision making. The guidelines should be applicable to unsignalized intersections (no control, yield control, and stop control on the minor road). It is expected that the research and guidelines will address the following elements:
1. Identification of appropriate definitions and methods to measure ISD;
2. Quantification of the relationship between safety and available ISD (i.e., Crash Modification Factors and other appropriate functions);
3. Guidance to transportation agencies on how to apply these functions to evaluate the safety impacts of available sight distance
Complete. NCHRP Research Report 875: Guidance for Evaluating the Safety Impacts of Intersection Sight Distance is a resource for practitioners involved in the planning, design, operations, and traffic safety management of stop-controlled intersections. It provides information on how to estimate the effect of ISD on crash frequency at intersections and describes data collection methods and analysis steps for making safety-informed decisions about ISD. The guidance also provides basic information on the importance of ISD that can be shared with decision makers and other stakeholders.
Accompanying the report, NCHRP Web-Only Document 228: Safety Impacts of Intersection Sight Distance documents the methodology and presents the results from the underlying research on estimating the safety effects of ISD at stop-controlled intersections. To establish the relationship between ISD and safety at stop-controlled intersections, crash, traffic, and geometric data were collected for 832 intersection approaches with minor-road stop control in North Carolina, Ohio, and Washington.