Analysis and evaluation of safety impacts of median types and midblock left turn treatments for urban arterials.

摘要

Urban growth leads to new land-uses abutting arterials requiring driveways for their accessibility. Uncontrolled number and locations of such access points causes safety, mobility and accessibility problems. The solution to these problems is access management (AM) which controls the number and location of the access points. AM techniques are normally documented in the form of guidelines for engineers and planners to follow when implementing the techniques. However, AM guidelines may not cover every technique due to the fact that AM is still growing. For example, the current AM guideline prepared by The Nevada Department of Transportation addresses many AM techniques. The guideline, however, addresses the design of lengths and ends of median openings but not spacing and type of the openings in segments with raised median (RM).;Spacing, location, and types of median openings have impacts on safety of midblock sections of arterials. Short spacing of median openings results in overlapping functional areas and consequently high number of traffic conflicts and crashes. Long spacing of median openings results in few median openings in a given segment length hence concentrating turning traffic at those few median openings. Concentrating turning traffic at the openings increases potential conflicts, impedance to through traffic, and accessibility problems. This study evaluates the impacts of median type, density, spacing, location, and type of median openings and proposes optimal spacings that minimize number of crashes.;This study deviates from past studies that evaluated safety impacts of an aggregate number of median openings using crash data collected over shorter periods of one to three years. The studies reported mixed results, making it difficult to transfer findings across geographical locations. Aggregating the impacts might have concealed the impacts of individual spacing between median openings.;Statistical models were calibrated for median openings in RM segments at aggregate and disaggregate levels of analysis. Other variables such as signal spacing, number of driveways, land-use, AADT, and speed limits were included.;Results of the analyses reveal that density, spacing, location and type of median openings do have significant impacts on midblock crashes. The results show that one median opening in a mile corresponds to 5.7% and 5.3% total and injury crash rates, respectively. Optimal spacing of the median openings is found in the range of 340 feet to 730 feet based on types of crashes and speed limits. Median openings located adjacent to signalized intersection have up to 30% more crashes than intermediate openings.;The results of this research are expected to assist transportation agencies in prioritizing retrofit projects, updating existing, and developing new AM strategies related to spacing between median openings.