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15 ans. Bonneson and McCoy (24) found raised-curb median ways under heavy volume conditions when the diversion dis- treatments to be associated with fewer accidents than tances are generally less than 0.8 km (0.5 mi). TWLTLs, especially for streets with average daily traffic In another study, conducted by Kach (30) in Michigan, the (ADT) volumes exceeding 20,000. To determine the opera- safety performance of directional median openings was com- tional and safety effects of replacing a TWLTL with a raised pared with that of conventional median openings to deter- median, the Gwinnett County Department of Transportation mine the safety benefits that can be attributed to prohibiting in Georgia compared the accident experience before and left turns from the minor road. The mean intersection-related after construction of a raised median (25). An accident analy- accident rates for directional median openings were found to sis determined that retrofitting a TWLTL with a 254-mm be 15 percent lower than for conventional median openings. (10-in) concrete raised median reduces accidents. Squires Similarly, the rates for intersection-related injury accidents and Parsonson (19) concluded that, on high-volume road- were 30 percent lower for directional median openings than ways, nontraversable medians have a lower crash experience for conventional median openings. The study also showed than roadways with continuous TWLTLs. An analysis by substantial reductions in right-angle, rear-end, left-turn, and Glennon et al. (26, 27, 28) found that TWLTLs have higher head-on accidents. In general, the results of this study indi- accident rates than raised medians where frequent driveways cate that directional median openings, where left turns are are found in combination with high arterial street volumes. prohibited on the minor road, carry higher volumes at a lower Glennon et al. also found the raised median to be a more accident rate than conventional median openings, where all effective technique under higher traffic volumes. Margiotta turns are permitted. and Chatterjee (29) performed a safety analysis of TWLTLs Levinson et al. analyzed the safety benefits of directional and raised medians. The study concluded that medians are versus conventional median openings as a function of traf- generally safer than TWLTLs, but certain conditions exist fic signal density for 123 segments constituting 364 km where TWLTLs would have a more favorable safety experi- (226 mi) of divided highway in Michigan (31). The authors ence. Regression analysis found that driveway density is an reported that directional median openings have one-third the important contributor to accident rates for medians, but not accident rate of TWLTLs and about two-thirds the accident for TWLTLs. rate of conventional median openings. The operational ben- efits also included increased capacity, reduced travel times, and improved signal coordination. SAFETY EFFECTS OF INCREASING U-TURN In a paper presented at the 1996 annual meeting of the MANEUVERS THROUGH USE OF Transportation Research Board (32), Maki reported the safety NONTRAVERSABLE MEDIANS results of replacing conventional median openings with direc- tional median openings in Michigan. The Michigan DOT was The installation of a nontraversable median may prevent experiencing capacity problems on an arterial because of many direct left-turn movements previously accessible to interlocking left turns within the conventional median open- motorists, forcing those motorists to use indirect routes and, thus, increasing the volume of U-turn maneuvers. The effect ings at major intersections. The results of replacing four con- of this increase in U-turn volumes on the safety of the roadway ventional median openings with directional median openings is not well understood. However, several studies have identi- were significant in reducing the number of crashes, particu- fied issues related to U-turns at unsignalized median openings. larly right-angle crashes. The average number of accidents Research by Gluck et al. in NCHRP Report 420: Impacts per year was reduced from 32 to 13--a decrease of about of Access Management Techniques (4) documents the safety 61 percent. Angle collisions were reduced by 96 percent, and operational experience in several states where directional sideswipe collisions by 61 percent, and rear-end collisions by U-turn median openings have replaced conventional median 17 percent. Injury accidents decreased by 75 percent. openings. The states reported that closing conventional Although the Florida Median Handbook (8) does not address median openings and replacing them with directional median specific safety issues related to U-turn maneuvers, it provides openings improves safety. Specifically, NCHRP Report 420 guidance on where a U-turn median opening should be con- indicates that eliminating direct left turns from driveways sidered. First, it lists several indicators that a U-turn median and replacing them with indirect U-turn maneuvers results in opening should be considered in advance of a signalized a 20-percent reduction in accident rate. U-turn crossovers intersection: were found to have roughly one-half of the accident rates of roads with TWLTLs. The operational analysis in NCHRP Level-of-service problems exist at the intersection. Report 420 identified several operational benefits of direc- Heavy left-turn movements are present at the signal. tional versus conventional median openings: shorter travel Heavy conflicting right-turn movements are present at times, less delay, and increased capacity. The report states the intersection. that right turns followed by U-turns can provide comparable, Gaps in oncoming traffic would be beneficial at a sepa- if not shorter, travel times than direct left turns from drive- rate U-turn opening.