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18 Figure 5. Examples of offset left-turn lanes (3). period. The research developed quantitative safety effective- Based on the results of this study, the AMFs for turn lanes in ness measures for installation design improvements involving Table 6 have been revised as shown in Table 7. added left-turn lanes and added right-turn lanes. The research concluded that added left-turn lanes are effective in improv- MEDIAN ACCELERATION LANES ing safety at signalized and unsignalized intersections in both rural and urban areas. More specifically, the research con- Median acceleration lanes are increasingly used at inter- cluded that sections on high-speed divided highways. They provide vehicles turning left onto a divided highway from a minor · Installation of a single left-turn lane on a major-road road with a path to accelerate to an appropriate speed before approach would be expected to reduce total intersection entering the through travel lanes. Median acceleration lanes accidents at rural unsignalized intersections by 28 per- provide both safety and operational benefits in that the enter- cent for four-leg intersections and by 44 percent for ing vehicles do not cause vehicles on the major roadway to three-leg intersections. decelerate substantially. · At urban unsignalized intersections, installation of a left- Median acceleration lanes can allow a full median open- turn lane on one approach would be expected to reduce ing to operate with some of the characteristics of a directional accidents by 27 percent for four-leg intersections and by median opening. Figure 6 illustrates a typical divided high- 33 percent for three-leg intersections. way intersection with median acceleration lanes. · At four-leg urban signalized intersections, installation In NCHRP Report 375 (7) four intersections with one or of a left-turn lane on one approach would be expected more median acceleration lanes were studied with accident to reduce accidents by 10 percent. field data. These studies found that median acceleration lanes
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19 TABLE 7 Accident modification factors for installation of left-turn lanes on the major- road approaches to intersections on two-lane rural highways (53) Number of major-road approaches on which Intersection left-turn lanes are installed Intersection type traffic control One approach Both approaches a Three-leg intersection STOP sign 0.56 b Traffic signal 0.85 a Four-leg intersection STOP sign 0.72 0.52 b b Traffic signal 0.82 0.67 a STOP signs on minor-road approach(es) b based on results in Reference 45 can enhance the operation of intersections on divided high- that median acceleration lane use was found to be limited, ways. In particular, median acceleration lanes reduce the there was insufficient information for a quantitative analysis. likelihood that vehicles making a left turn from a crossroad However, some of the advantages and disadvantages approach will need to stop in the median. reported by the agencies include the following: Encroachment on the through lanes of a divided highway with a narrow median is a particular problem when larger Advantages: vehicles are forced to stop in the median opening area. NCHRP Synthesis of Highway Practice 281: Operational · Median acceleration lanes reduce delays when traffic vol- Impacts of Median Width on Larger Vehicles (54), recom- umes are high. mends the provision of median acceleration lanes to mini- · Median acceleration lanes provide higher merging speeds. mize the likelihood that larger vehicles will be required to · Median acceleration lanes are useful if the acceleration stop in the median opening area. Median acceleration lanes lane is long enough to allow a safe merge. normally allow vehicles turning left onto the divided high- · Median acceleration lanes reduce accidents. way to proceed without stopping, except when conflicting traffic is present in the median opening area at the same time. Disadvantages: Furthermore, the acceleration lane permits larger vehicles, which accelerate slowly, to enter the through lanes of the · It is difficult to merge from median acceleration lanes divided highway at a higher speed. This should minimize the because of blind spots. potential for collisions between through and turning vehicles. · Median acceleration lanes are not used properly by In 1982, ITE conducted a survey of highway and traffic drivers. agencies in Canada and the United States concerning current · Median acceleration lanes create anxiety to through use of median acceleration lanes and TWLTLs (38). Given traffic. Figure 6. Typical divided highway intersection with median acceleration lanes (7).