Safety of U-Turns at Unsignalized Median Openings (2004) / Chapter Skim
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Pages 5-26
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From page 5... ...
Some factors that influence median opening locations include the following: • Spacing between median openings, • Stopping sight distance, • Intersection sight distance, • Operating speeds, • Length of turn lanes, • Right-turn conflict overlap, and • Size and type of traffic generator. A committee of the Institute of Transportation Engineers developed a list of factors to consider in locating median openings (1)
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From page 6... ...
The report also presents minimum desired spacing of unsignalized median openings at driveways as a function of speed. This spacing ranges from 113 m (370 ft)
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From page 7... ...
found that very few state highway agencies have formal policies on the minimum spacing between median openings. Those agencies that do have criteria generally use a spacing between median openings in the range from 0.4 to 0.8 km (0.25 to 0.5 mi)
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From page 8... ...
in NCHRP Report 375 (7) found that the median width and median opening length have a strong influence on the safety performance of median openings.
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From page 9... ...
The field observational study in NCHRP Report 375 investigated the effect of median width on three types of undesirable driving behavior as commonly observed in the median opening area at intersections on divided highways: • Encroachment on through lanes by vehicles in the median opening area, • Side-by-side queuing of vehicles in the median opening area, and • Angle stopping by vehicles in the median opening area. Figure 3 illustrates side-by-side queuing of vehicles in an unsignalized median opening.
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From page 10... ...
-- single left turns 12.6 (42.0) -- dual left turns Recommended Six-lane highways with medians expecting significant U-turns and directional median openings with excellent positive guidance All 7.0 (22.0)
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From page 11... ...
The accident rate comparisons from the various studies are summarized in Table 4. In 15 out of the 16 comparisons shown in Table 4, the accident rates were reduced when a nontraversable median was installed in place of a TWLTL.
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From page 12... ...
TABLE 3 Comparison of safety performance of alternative midblock cross-section as reported by studies in the literature (15)
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From page 13... ...
TABLE 4 Synthesis of safety experience comparing TWLTLs with nontraversable medians by percent difference (4)
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From page 14... ...
median crossovers for unsignalized sections of highways. Signalized divided highways with directional crossovers were found to have about 50 percent of the accident rate of similar facilities with conventional median openings.
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From page 15... ...
in Michigan, the safety performance of directional median openings was compared with that of conventional median openings to determine the safety benefits that can be attributed to prohibiting left turns from the minor road. The mean intersection-related accident rates for directional median openings were found to be 15 percent lower than for conventional median openings.
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From page 16... ...
. The manual for the course recommends installing left-turn lanes at existing median openings to • Allow turning vehicles to clear the through traffic lane with an acceptable speed differential, • Provide queue storage without interference with through traffic, • Reduce rear-end collisions and sideswipes, and • Increase capacity and decrease delay.
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From page 17... ...
Although offset left-turn lanes have been used primarily at signalized median openings, they have been used by at least two agencies at unsignalized median openings. Offset left-turn lanes are a potential concern because they may make U-turn maneuvers more difficult to complete because they move the starting point for the U-turn maneuver closer to the opposing roadway.
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From page 18... ...
Median acceleration lanes can allow a full median opening to operate with some of the characteristics of a directional median opening. Figure 6 illustrates a typical divided highway intersection with median acceleration lanes.
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From page 19... ...
, recommends the provision of median acceleration lanes to minimize the likelihood that larger vehicles will be required to stop in the median opening area. Median acceleration lanes normally allow vehicles turning left onto the divided highway to proceed without stopping, except when conflicting traffic is present in the median opening area at the same time.
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From page 20... ...
Median acceleration lanes can improve the operation of directional median openings by helping U-turning vehicles to accelerate and merge with traffic on the through roadway. There are no data on whether median acceleration lanes at conventional median openings create additional conflicts for drivers making U-turn maneuvers.
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From page 21... ...
60 160 70 200 80 260 90 380 100 470 Assumptions: Design vehicle = passenger vehicle Reaction time = 2.0 sec Extra time needed in the U-turn maneuver U-turn vehicle begins acceleration from 0 mph only at the end of the U-turn movement Values are based on speed/distance/acceleration figures from the 1990 AASHTO Green Book 50-ft clearance factor TABLE 8 Sight distance for U-turns at unsignalized median openings (8)
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From page 22... ...
. Several design alternatives are presented.
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From page 23... ...
The report states that although the denial of left-turn access by a raised median is likely to increase U-turn demand at nearby median openings, it is also likely that some larger vehicles will use indirect routes that do not involve a U-turn maneuver to reach their destination. Such routes may involve going around the block or may 23 incorporate an entirely different route from origin to destination, so that the larger vehicle can make a right turn into the driveway at its destination.
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From page 24... ...
A consid24 erable amount of literature focuses on access management issues, and several studies have evaluated the relationship between safety and access management. Although none of these studies specifically address the safety at unsignalized median openings, the research that has established relationships between access density and safety can be useful in this study as well.
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From page 25... ...
The need to address the safety effects of U-turns at unsignalized median openings is a direct result of increased attention to access management. Highway agencies are installing more raised medians on arterials in response to access management guidelines.
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From page 26... ...
addressed queue spillbacks and states that median openings should not be placed across regularly forming queues from neighboring intersections. Median openings placed too closely to an intersection cause both safety and operational problems.
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