Safety of U-Turns at Unsignalized Median Openings (2004) / Chapter Skim
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Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings
Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings
Pages 106-133
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From page 106... ...
The following discussion addresses various types of unsignalized median openings and guidelines for their use and design. CLASSIFICATION AND ASSESSMENT OF TYPICAL MEDIAN OPENING DESIGNS A classification system is presented to identify how particular median openings function and where they are located relative to other elements of the highway system.
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From page 107... ...
Estimated accident rates by type of median-urban and suburban areas.
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From page 108... ...
Conventional Median Opening at Four-Leg Intersection 6. Directional Median Opening at Four-Leg Intersection These six categories of median openings can be subdivided based on the presence of left-turn lanes or loons and the types of turning maneuvers permitted.
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From page 109... ...
Conventional median openings are appropriate where U-turn volumes are relatively low, such that U-turn vehicles in opposing directions of travel create minimal interference with one another. The conventional midblock median opening design is further classified into three subcategories based on the presence of left-turn lanes and/or loons: Type 1a -- Conventional Midblock Median Opening Without Left-Turn Lanes Type 1b -- Conventional Midblock Median Opening With Left-Turn Lanes C-4 Type 1c -- Conventional Midblock Median Opening With Left-Turn Lanes and Loons Figures C-3 through C-5 illustrate these three median opening designs and their advantages and disadvantages.
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From page 110... ...
• Providing median openings for U-turns between intersections reduces the number of turning maneuvers at the intersections. • Accident rates at midblock median openings are lower than at three- or four-leg median openings.
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From page 111... ...
Median openings at three-leg intersections are appropriate along arterial roadways at street intersections or driveways to major developments where providing access across the median will not create undesirable safety or traffic operational effects. Conventional median openings are appropriate where it is desirable to allow left-turning movements from both the major road and the minor road (or driveway)
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From page 112... ...
• Providing median openings for U-turns between intersections reduces the number of turning maneuvers at the intersections. • Accident rates at midblock median openings are lower than at three- or four-leg median openings.
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From page 113... ...
Median openings at four-leg intersections are appropriate along arterial roadways at street intersections or driveways to major developments where providing access across the median will not create undesirable safety or traffic operational effects. Conventional median openings are appropriate where it is desirable to allow left-turning movements from both the major road and the minor road (or driveway)
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From page 114... ...
Directional median openings are appropriate where U-turn or left-turn volumes are relatively high, such that a conventional median opening would experience considerable interference between vehicles entering the median opening. This particular directional median opening design is also appropriate where there is a disproportionately high leftturn demand from the major road.
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From page 115... ...
, as discussed in the next section. U-turn and Left-Turn Volumes Field studies at a variety of median opening types in urban arterial corridors have found estimated U-turn volumes rangType 3a -- Conventional Median Opening Without Left-Turn Lanes at Three-Leg Intersection Advantages • Vehicles on the minor road can make both left and right turns onto the major road.
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From page 116... ...
However, Table C-2 presents the accident rates per median opening traffic movement for various area and median opening types. The median opening traffic volume is the sum of the U-turn and left-turn volumes through the median opening.
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From page 117... ...
They not only increase conflicts with and delays to other vehicles, but also pose a major safety problem with the large speed differential between left-turning and through vehicles. Type 3c -- Conventional Median Opening With Two Left-Turn Lanes at Three-Leg Intersection • The presence of left-turn lanes reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for both directions of travel on the major road.
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From page 118... ...
• Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. • The presence of a loon may make snow removal and other maintenance work more difficult.
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From page 119... ...
• There are only six conflict points, which is less than at a conventional median opening at a three-leg intersection. • Accident rates at three-leg intersections are lower than at four-leg intersections.
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From page 120... ...
By contrast, at median openings in urban and suburban areas, the median width should not be Type 4b -- Directional Median Opening for Left Turns Onto Major Road at Three-Leg Intersection Advantages • Vehicles on the minor road have direct left-turn access to the major road. • Vehicles on the major road in one direction of travel can make U-turn maneuvers.
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From page 121... ...
U-turn maneuType 5a -- Conventional Median Opening Without Left-Turn Lanes at Four-Leg Intersection Advantages • Vehicles on the minor road can either cross the major road or make left and right turns onto the major road. • Vehicles on the major road in both directions of travel can make left and right turns onto the minor road and make U-turns on the major road.
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From page 122... ...
. The following ISD cases are applicable to unsignalized median openings: • Case B1 (left turns from the minor road)
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From page 123... ...
• There are only 12 conflict points, which is less than at a conventional median opening at a four-leg intersection. • Accident rates at directional four-leg median openings are lower than at conventional four-leg median openings.
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From page 124... ...
Specifically, while median opening Type 2c was found to have a higher average median opening accident rate than median opening Type 2a, the individual accident patterns at these sites were reviewed; it was confirmed that the accidents at median openings of Type 2c did not involve trucks and were not related to loon usage. Although the sample size is very limited, there is no indication that provision of loons or their use by large trucks leads to safety problems.
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From page 125... ...
Typical loon design at a directional median opening (55, 56)
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From page 126... ...
An overview of the methodology is presented first, followed by a detailed discussion of the methodology and two examples with sample calculations. Overview of Methodology The methodology for comparing the expected safety performance of median opening design alternatives consists of five logically sequenced steps as follows: Step 1: Select median opening design alternatives Step 2: Enter all turning volumes Step 3: Enter the average non-intersection accident rate for the major road Step 4: Calculate the accident rate for each median opening design alternative Step 5: Compare accident rates for each median opening design alternative Methodology Step 1: Select median opening design alternatives Typically, highway agencies are faced with selecting the most appropriate median opening design from a number of alternatives.
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From page 127... ...
Step 3: Enter the average non-intersection accident rate for the major road If any of the alternatives identified in Step 1 represents a combination of median opening designs, the average nonintersection accident rate for the major road is needed. This may be obtained from actual accident data or may be estimated based on historical accident experience or engineering judgment.
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From page 128... ...
Step 4c: Calculate the total accident rate for each median opening design alternative For alternatives that consist of a single median opening, the total accident rate is equal to the accident rate of that individual median opening, as computed in Step 4a. For alternatives that consist of a combination of median openings, the total accident rate is equal to the accident rates for each individual median opening, as computed in Step 4a, and the accident rate for exposure related to additional travel distance, as computed in Step 4b.
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From page 129... ...
For example, if one alternative accommodates fewer turning movements than another alternative, selecting that alternative may have a negative impact on the traffic operational performance of the arterial corridor. Example 1: Comparison of the safety performance of conventional and directional median openings at three-leg intersections Step 1: Select median opening design alternatives Alternative 1: Conventional median opening at a threeleg intersection Alternative 2: Directional median opening at a three-leg intersection in combination with a directional midblock median opening Figure C-21 illustrates Alternatives 1 and 2.
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From page 130... ...
Directional three-leg: The accident rate for a directional median opening at a three-leg intersection is computed as follows: From Table C-2, the median opening accident rate for a directional three-leg median opening is 1.40 accidents per million turning vehicles. The total turning volume for this median opening is 250 veh/day, since the only turning movements permitted include left turns from the major road onto the minor road (Direction 2)
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From page 131... ...
In Alternative 2, the directional median opening at the four-leg intersection accommodates vehicles making left-turn maneuvers C-26 from the major road onto the minor road and vehicles making U-turns on the major road. Left-turn vehicles from the minor road approaches must use the directional midblock median openings to complete their turning maneuvers.
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From page 132... ...
= ___ (acc/yr) Total accident rate = 1.10 acc/yr Alternative 2: Combination of Directional Median Opening at a Four-Leg Intersection and Two Directional Midblock Median Openings Accident Rate acc/yr mi( )
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From page 133... ...
= 0.24 acc/yr Total accident rate = 0.69 acc/yr C-28 Step 5: Compare accident rates for each median opening design alternative Alternative 1 = 1.10 acc/yr Alternative 2 = 0.69 acc/yr In this example, the combination of directional median openings (Alternative 2) represents a 32% reduction in accident rate over the conventional median opening at a fourleg intersection (Alternative 1)
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