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53 physically separated rights-of-way by roadways, bike paths, on six-lane highways at eight sites in Florida. After analyz- or pedestrian facilities. TCRP Report 117 (Eccles et al. 2007) ing the data, the authors concluded that U-turns could have documents the activities on that project, and the guidance better operational performance than DLTs under certain contained in that report is intended to "provide information traffic conditions, which they said implied that directional that can be applied to enhance safety at busway crossings median opening designs would provide more efficient traffic while maintaining efficient transit and highway operations flow than full median openings. They also stated that RTUT and minimizing pedestrian delay." General design principles would provide better safety with regard to traffic conflicts and guidelines included: and fewer effects on through-traffic operations of a major highway; they added that the majority of traffic on the major · Provide simple intersection designs. street in the study was in platoon flow because the signal · Provide clear visual cues to make busway intersections spacing at study sites was less than 2 mi. Turns could not be conspicuous. made when the platoon was passing the driveway, and strag- · Maximize driver and pedestrian expectancy. glers and left-turn-in movements from major roads between · Separate conflicting movements. the platoons affected the ability to make turns. · Minimize street crossings. · Incorporate design features that improve safety for vul- Carter et al. (2005a) examined the operational and safety nerable users. effects of U-turns at signalized intersections. The operational · Coordinate geometric design features and traffic control analysis involved measurements of vehicle headways in exclu- devices. sive left-turn lanes at 14 signalized intersections. Regression analysis of saturation flow data showed a 1.8% saturation flow TCRP Report 117 discussed four types of busways found rate loss in the left-turn lane for every 10% increase in U-turn at intersections: median busways, side-aligned busways, sep- percentage and an additional 1.5% loss for every 10% U-turns arated right-of-way busways, and bus-only ramps. For each if the U-turning movement was opposed by protected right- busway type, the report contains guidance on safety issues, turn overlap from the cross street. The safety analysis involved basic geometry (including placement of bus stops), and traf- a set of 78 intersections, 54 sites chosen randomly and 24 fic control, as well as an example of an intersection that uses sites selected on the basis of their reputation as U-turn problem each type of busway. Safety issues were generally related to sites. Although the researchers used a group of study sites that the complexity and/or unfamiliarity of the arrangement of the was biased toward sites with high U-turn percentages, they intersection and the accommodation of pedestrians. Geometry found that 65 of the 78 sites did not have any collisions involv- guidelines pertained to channelization and control of turning ing U-turns in the 3-year study period. U-turn collisions at the movements to protect buses and passengers, provision of suf- remaining 13 sites ranged from 0.33 to 3.0 collisions per year. ficient right-of-way to include the number of necessary travel Sites with double left-turn lanes, protected right-turn overlap, lanes, and providing design consistency between busway or high left-turn and conflicting right-turn traffic volumes were lanes and the adjacent general-purpose lanes. The traffic con- found to have a significantly greater number of U-turn col- trol device most frequently recommended was traffic signals; lisions. Researchers concluded that, overall, U-turns do not suggestions on timing and phasing were provided to promote have the large negative effect at signalized intersections that optimization of capacity and safety. many have assumed, as safety and operational effects were minimal. Access Management In NCHRP Project 17-21, researchers determined state and local agency design practices and policies related to unsig- Recent research has established support for use of access nalized median openings for U-turns, such as those shown management principles in improving intersection design and in Figure 20. After seven categories of midblock and inter- safety. The optimal situation is to avoid driveway conflicts section median designs were identified, the research team before they develop (Neuman et al. 2003b). "This requires assessed the designs' effects on safety through field obser- coordination with local land use planners and zoning boards vation and crash data analysis for 115 unsignalized median in establishing safe development policies and procedures. opening sites with both crash and field data. This knowledge Avoidance of high-volume driveways near congested or other- was transferred into design guidelines and a methodology wise critical intersections is desirable. Driveway-permitting for comparing the expected safety performance of different staff within highway agencies also needs to have an under- designs, to enable engineers in setting policy, establishing standing of the safety consequences of driveway requests." project-level design, and discussing the impacts of medi- Some recent research, findings, and discussion related to access ans with business and property owners. As documented in management are contained in this section. NCHRP Report 524 (Potts et al. 2004), researchers made the following conclusions: Zhou et al. (2002) studied the operational effectiveness of using right-turn-plus-U-turn (RTUT) as an alternative to DLTs · As medians are used more extensively on arterial high- from driveways where raised-curb medians were installed ways, with direct left-turn access limited to selected
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54 median opening. Therefore, researchers concluded that design decisions consider the relative safety and opera- tional efficiency of all directional median openings in comparison with the single conventional median opening. · 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. · At urban unsignalized intersections, the research found that installation of a left-turn lane on one approach would be expected to reduce accidents by 27% for four-leg intersections and by 33% for three-leg intersections. · The minimum spacing between median openings then used by highway agencies ranged from 152 to 805 m (500 to 2,640 ft) in rural areas and 91 to 805 m (300 to 2,640 ft) in urban areas. In most cases, highway agen- FIGURE 20 Median opening for left-turn lane on a four-lane cies used spacings between median openings in the divided suburban arterial at an unsignalized three-leg intersection (Credit : Dan Walker, Texas Transportation Institute). upper end of these ranges, but there was no indication that safety problems resulted from occasional use of median opening spacings as short as 91 to 152 m (300 locations, many arterial highways experience fewer mid- to 500 ft). block left-turn maneuvers and more U-turn maneuvers at unsignalized median openings. Based on these and other findings, supplemented in part · Field studies at various median openings in urban arterial by conclusions in NCHRP Report 348 (Koepke and Levinson corridors found estimated U-turn volumes of no more 1992), NCHRP Report 375 (Harwood et al. 1995), and NCHRP than 3.2% of the major-road traffic volumes at those Report 420 (Gluck et al. 1999), the NCHRP 17-21 research locations. At rural median openings, U-turn volumes team developed and presented a five-step methodology for were found to represent at most 1.4% of the major-road comparing the expected safety performance of median open- traffic volumes at those locations. ing design alternatives to assist in the selection of median · Accidents related to U-turn and left-turn maneuvers opening types and the comparison of alternative median open- at unsignalized median openings occurred very infre- ing arrangements. As part of their conclusions, the research quently. The 103 median opening study sites on urban arterial corridors experienced an annual U-turn plus team recommended the following: left-turn crash average of 0.41. Twelve median open- ings on rural arterial corridors had an annual average · Unsignalized median openings may be used for a broad crash total of 0.20. Overall, at these median openings, range of major- and minor-road traffic volumes. How- U-turns represented 58% of the median opening move- ever, if the major- and minor-road volumes exceed the ments and left turns represented 42%. Based on these traffic volumes given in the MUTCD signalization war- limited crash frequencies, researchers concluded that rants, signalization of the median opening needs to be there was no indication that U-turns at unsignalized considered. median openings constituted a major safety concern. · The effects of U-turn and left-turn volumes on median · For urban arterial corridors, median opening crash rates opening crash frequency cannot be separated, because were substantially lower for midblock median openings a review of crash data for median openings found that than for median openings at three- and four-leg inter- crash report data do not distinguish clearly between sections. For example, the crash rate per million median crashes involving U-turn maneuvers and those involv- opening movements (U-turn plus left-turn maneuvers) ing left-turn maneuvers. at a directional midblock median opening was typically · For rural unsignalized intersections: only about 14% of the median opening crash rate for a They should have medians that are as wide as practical, directional median opening at a three-leg intersection. as long as the median is not so wide that approaching · Crash rates at directional median openings on urban arte- vehicles on the crossroad cannot see both roadways of rial corridors were lower than at traditional median open- the divided highway. ings, and conventional three-leg median openings had Where the AASHTO passenger car is used as the lower crash rates than corresponding four-leg openings. design vehicle, a minimum median width of 8 m · Where directional median openings were considered (25 ft) is recommended. as alternatives to conventional median openings, two or Where a large truck is used as the design vehicle, a more directional median openings were usually required median width of 21 to 31 m (70 to 100 ft) generally to serve the same traffic movements as one conventional would be selected. If such a median width cannot be