The results of research involving a well-designed before-and-after evaluation of the safety effects of providing left- and right-turn lanes for at-grade intersections are presented. Geometric design, traffic control, traffic volume, and traffic accident data were gathered for a total of 280 improved intersections as well as 300 similar intersections that were not improved during the study period. The types of improvement projects evaluated included installation of added left-turn lanes, added right-turn lanes, and extension of the length of existing left- or right-turn lanes. An observational before-and-after evaluation of these projects was performed by using several alternative evaluation approaches. Three contrasting approaches to before-and-after evaluation were used: the yoked comparison or matched-pair approach, the comparison group approach, and the empirical Bayes approach. The research not only evaluated the safety effectiveness of left- and right-turn lane improvements but also compared the performances of these three alternative approaches in making such evaluations. The research developed quantitative safety effectiveness measures for installation design improvements involving added left-turn lanes and added right-turn lanes. The research concluded that the empirical Bayes method provides the most accurate and reliable results. Further use of this method is recommended.

The need for changes in the criteria for passing sight distance (PSD) used for design of two-lane highways and for marking of passing and no-passing zones on two-lane highways was investigated. PSD models from the AASHTO Green Book and the Manual on Uniform Traffic Control Devices were first compared with models developed in previous research to determine the most accurate representation of the passing maneuver. Two existing models warranted further consideration. Field studies were then conducted in Missouri and Pennsylvania; recent field data from a study in Texas were also evaluated to validate the assumptions made in the passing models. On the basis of current driver behaviors, no change was needed in the PSD criteria used for marking passing and no-passing zones. The research found that two-lane highways could be safely designed with PSD criteria equal to or greater than the PSD criteria used in marking passing and no-passing zones, instead of the longer PSD values currently used in design. Therefore, the use of the current PSD marking criteria in design is recommended. This practice will provide desirable consistency between PSD design and marking practices.

This research investigates the relationship between lane width and safety for roadway segments and intersection approaches on urban and suburban arterials. The research found no general indication that the use of lanes narrower than 3.6 m (12 ft) on urban and suburban arterials increases crash frequencies. This finding suggests that geometric design policies should provide substantial flexibility for use of lane widths narrower than 3.6 m (12 ft). Inconsistent results were found which suggested increased crash frequencies with narrower lanes in three specific design situations. Narrower lanes should be used cautiously in these three specific situations unless local experience indicates otherwise.

Congestion on urban freeways often creates a need to increase freeway capacity by adding an additional lane. Although adding a lane by widening the existing roadbed is often difficult and expensive, restriping the traveled way with narrower lanes, converting all or part of the shoulder to a travel lane, or a combination of both often is a practical solution. An observational before-and-after evaluation with the empirical Bayes method was done to examine the safety effects of projects involving narrower lanes or shoulder conversions on existing urban freeways in California with four or five lanes in one direction of travel. The evaluation found that projects converting four lanes to five lanes resulted in increases of 10% to 11% in accident frequency. Projects converting five lanes to six lanes resulted in smaller increases in accident frequency. These increases in accident frequency may be the result of accident migration caused by relocation of traffic operational bottlenecks.

FOREWORDThe objectives of this research are to synthesize the current state of knowledge concerning safety assessment of new or modified interchanges; develop a spreadsheet-based computational tool for performing safety assessments of interchanges; and identify gaps in knowledge concerning interchange safety assessment and future research needs to fill those gaps. The primary deliverables of this research are (1) the spreadsheet-based analytical tool [i.e., Interchange Safety Analysis Tool (ISAT)] which provides design and safety engineers with a means for assessing the safety effects of new and existing interchanges, (2) a user manual (FHWA-HRT-07-045) which provides detailed descriptions/instructions on the usage and operation of ISAT, and (3) this final report. This report summarizes the capabilities of ISAT, summarizes safety data related to interchanges, summarizes safety literature related to interchanges, presents results of telephone interviews with key stakeholders to investigate their needs and requirements in safety evaluations of interchanges, and identifies substantive gaps in the current state of knowledge that limit the ability of ISAT to provide all of the capabilities desired by potential users and provides a brief summary of the needed research to address these gaps. The report also contains the detailed algorithms utilized in the calculations of the ISAT program. Michael Trentacoste Director, Office of SafetyResearch and Development Notice This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document.The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this report only because they are considered essential to the objective of the document. Quality Assurance StatementThe Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement. The objectives of this research are to synthesize the current state of knowledge concerning the safety assessment of new or modified interchanges; develop a spreadsheet-based computational tool for performing safety assessments of interchanges; and identify gaps in knowledge concerning interchange safety assessment and future research needs to fill those gaps. This report summarizes the capabilities of the spreadsheet-based Interchange Safety Analysis Tool (ISAT), summarizes safety data related to interchanges, summarizes safety literature related to interchanges, presents results of telephone interviews with key stakeholders to investigate their needs and requirements in safety evaluations of interch...