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OCR for page 1
SAFETY OF U-TURNS AT UNSIGNALIZED MEDIAN OPENINGS SUMMARY The objective of this research was to determine the safety and operational effect of U-turns at unsignalized median openings. The safety performance of typical median opening designs were documented, and guidelines for the use, location, and design of unsignalized median openings were developed. The research scope included unsignal- ized median openings on all types of divided highways, but the focus of the research was on urban/suburban arterials because these present the greatest current challenge to highway agencies in access management. A catalog of median opening designs representative of the designs that actually exist in the field was created. The catalog included 918 unsignalized median openings that were found in 62 arterial corridors located in seven states. Median openings were clas- sified by type of geometry (conventional versus directional), number of intersection legs (midblock versus three-leg versus four-leg), presence of left-turn lane(s), and pres- ence of loon(s), resulting in a total of 17 typical median opening designs. Field studies to document how drivers behave in making U-turns and left turns at unsignalized median openings were conducted at 26 urban sites; supplementary man- ual traffic counts were also made at 77 median openings on urban arterials. In addition, field studies and/or manual traffic counts were made at 12 median openings on rural arterials. The primary field studies were conducted by videotaping traffic operations at selected median openings. Over 150 hours of videotape were reviewed to determine traffic volumes and to document traffic conflicts at various unsignalized median open- ing designs. Analysis of field data found that, for most types of median openings, most observed traffic conflicts involved major-road through vehicles having to brake for vehicles turning from the median opening onto the major road; however, for median openings at four-leg intersections without left-turn lanes on the major road, most of the observed traffic conflicts involved major-road through vehicles having to brake for vehicles turning left into the median opening. Accident studies of existing median openings were conducted to determine the rel- ative safety performance of median openings of various types. Out of 7,717 median- opening-related accidents, only 1% were identified as involving U-turns. However, it was also found that many accidents coded by the investigating officer as involving left-turn maneuvers, in fact, involved U-turn maneuvers. For this reason, accidents involving both U-turn and left-turn maneuvers had to be evaluated as a group.
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2 The research results indicate that access management strategies that increase U-turn volumes at unsignalized median openings can be used safely and effectively. Analy- sis of accident data found that accidents related to U-turn and left-turn maneuvers at unsignalized median openings occur very infrequently. In urban arterial corridors, unsignalized median openings experienced an average of 0.41 U-turn plus left-turn acci- dents per median opening per year. In rural arterial corridors, unsignalized median open- ings experienced an average of 0.20 U-turn plus left-turn accidents per median opening per year. Based on these limited accident frequencies, there is no indication that U-turns at unsignalized median openings constitute a major safety concern. Because of the low median opening accident frequencies, no satisfactory regression relationships relating median opening accident frequency to the volume of U-turn and left-turn maneuvers through the median opening could be developed. For urban arterial corridors, median opening accident rates are substantially lower for midblock median openings than for median openings at three- and four-leg inter- sections, and median opening accident rates are slightly lower for conventional three- leg median openings than for conventional four-leg median openings. Average median opening accident rates for directional three-leg median openings are about 48 percent lower than for conventional three-leg median openings, and average median opening accident rates for directional four-leg median openings are about 15 percent lower than for conventional four-leg intersections. The report recommends that midblock median openings be considered, where appro- priate, as a supplement or an alternative to median openings at three-leg or four-leg intersections. It is also recommended that directional median openings at three- or four- leg intersections, combined with directional midblock median opening(s), be consid- ered as a supplement or an alternative to conventional median openings at three- or four-leg intersections. The report presents guidelines for the use, location, and design of unsignalized median openings. The guidelines include a methodology for comparing the relative safety per- formance of alternative median opening designs.