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79 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS This chapter presents the conclusions of the research and recommendations for future work. CONCLUSIONS The conclusions of the research are as follows: ⢠As medians are used more extensively on arterial high- ways, with direct left-turn access limited to selected locations, many arterial highways experience fewer mid- block left-turn maneuvers and more U-turn maneuvers at unsignalized median openings. ⢠Field studies at various median opening types in urban arterial corridors have found estimated U-turn volumes ranging from 2 to 977 veh/day, representing from 0.01 to 3.2 percent of the major-road traffic volumes at those locations. At median openings in rural areas, U-turn vol- umes were found to range up to 374 veh/day, represent- ing at most 1.4 percent of the major-road traffic volumes at those locations. ⢠Review of accident data for median openings at which U-turn maneuvers are made have found that accident report data do not distinguish clearly between accidents involving U-turn maneuvers and those involving left- turn maneuvers. In particular, at some median openings where U-turn maneuvers can be made but no left-turn maneuvers are feasible, investigating officers classified a substantial proportion of the accidents involving turn- ing movements through the median as related to left- turn maneuvers. For this reason, reliable evaluation of median opening accident frequency must necessarily consider the frequency of U-turn-related and left-turn- related accidents combined. ⢠Accidents related to U-turn and left-turn maneuvers at unsignalized median openings occur very infrequently. The 103 median openings in urban arterial corridors eval- uated in detail in this research experienced an average of 0.41 U-turn plus left-turn accidents per median open- ing per year. The 12 median openings in rural arterial corridors evaluated in detail in this research experienced an average of 0.20 accidents per median opening per year. Overall, at these median openings, U-turns repre- sent 58 percent of the median opening movements and left turns represent 42 percent of the median opening movements. Based on these limited accident frequen- cies, there is no indication that U-turns at unsignalized median openings constitute a major safety concern. ⢠Average accident rates per median opening movement (U-turn plus left-turn maneuvers) have been estimated for specific median opening types in both urban and rural arterial corridors. No satisfactory regression rela- tionships relating median opening accident frequency to the volume of U-turn and left-turn maneuvers through the median opening could be developed. Given the low accident frequencies, this is not surprising. ⢠For urban arterial corridors, median opening accident rates are substantially lower for midblock median open- ings than for median openings at three- and four-leg intersections. For example, the accident rate per mil- lion median opening movements (U-turn plus left-turn maneuvers) at a directional midblock median opening is typically only about 14 percent of the median opening accident rate for a directional median opening at a three- leg intersection. ⢠For urban arterial corridors, average median opening acci- dent rates are slightly lower for conventional three-leg median openings than for conventional four-leg median openings ⢠For urban arterial corridors, average median opening accident rates for directional three-leg median openings are about 48 percent lower than the accident rates for conventional three-leg median openings. ⢠For urban arterial corridors, average median opening accident rates for directional four-leg median openings are about 15 percent lower than for conventional four- leg intersections. ⢠For rural arterial corridors, the average median open- ing accident rate is lower for median openings at three- leg intersections than for median openings at four-leg intersections. However, the sample size of median open- ings and median-opening-related accidents for rural arterial corridors is so small that no firm conclusions can be drawn. ⢠Where directional median openings are considered as alternatives to conventional median openings, two or more directional median openings are usually required
to serve the same traffic movements as one conventional median opening. Therefore, design decisions should con- sider the relative safety and operational efficiency of all directional median openings in comparison with the sin- gle conventional median opening. ⢠A comparison of the total median opening accident rates for the range of median opening types considered in the research is presented in Table 35. Comparable results for fatal-and-injury and property-damage-only accident rates at median openings are presented in Tables 36 and 37. ⢠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 turn- ing from the median opening onto the major road. ⢠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. ⢠The minimum spacing between median openings cur- rently used by highway agencies in rural areas ranges from 152 to 805 m (500 to 2,640 ft). In urban areas, the minimum spacing between median openings ranges from 91 to 805 m (300 to 2,640 ft) in highway agency policies. In most cases, highway agencies use spacings between median openings in the upper end of these ranges, but there is no indication that safety problems result from occasional use of median opening spacings as short as 91 to 152 m (300 to 500 ft). RECOMMENDATIONS The following recommendations have been developed based on these conclusions: ⢠Guidelines for the use, location, and design of unsignal- ized median openings are included in Appendix C of this report. ⢠The guidelines include tables presenting the advantages and disadvantages of typical median opening designs; these tables should be used by designers as a resource when considering alternative median opening designs. ⢠A methodology for comparing the expected safety per- formance of median opening design alternatives, as pre- sented in Appendix C, should be used to assist in the selection of median opening types and the comparison of alternative median opening arrangements. 80 ⢠Unsignalized median openings may be used for a broad range of major- and minor-road traffic volumes. However, if the major- and minor-road volumes exceed the traffic volumes given in the MUTCD signalization warrants, sig- nalization of the median opening should be considered. ⢠When evaluating the safety performance of unsignal- ized median openings, highway agencies should take into account both U-turn and left-turn maneuvers because accident report data do not distinguish clearly between accidents involving U-turn maneuvers and those involv- ing left-turn maneuvers. ⢠Median widths at suburban unsignalized intersections generally should be as narrow as possible while provid- ing sufficient space in the median for the appropriate left-turn treatment and to accommodate U-turn maneu- vers by a selected design vehicle. ⢠Rural unsignalized intersections should have medians that are as wide as practical, as long as the median is not so wide that approaching vehicles on the crossroad can- not see both roadways of the divided highway. ⢠Median opening lengths at urban and suburban divided highway intersections may be as long as necessary. In contrast, median opening lengths at rural divided high- way intersections generally should be kept to the mini- mum possible. ⢠Intersection sight distance based on the criteria in the AASHTO Green Book (3) for Cases B1, B2, and F should be available to accommodate U-turns and left turns at unsignalized median openings. ⢠Where a large truck is used as the design vehicle for a median opening and a median width of 21 to 31 m (70 to 100 ft) cannot be provided, consideration should be given to providing a loon. ⢠Left-turn lanes on the major road are desirable to mini- mize conflicts between through and turning vehicles at unsignalized median openings. ⢠Midblock median openings should be considered, where appropriate, as a supplement or an alternative to median openings at three-leg or four-leg intersections. ⢠Directional median openings at three-leg intersections, combined with a directional midblock median opening, should be considered as a supplement or an alternative to conventional median openings at three-leg intersections. ⢠Directional median openings at four-leg intersections, combined with two directional midblock median open- ings, should be considered as a supplement or an alter- native to conventional median openings at four-leg intersections.
