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From page 33... ... This classification system identifies how particular median openings function and where they are located relative to other elements of the highway system. This chapter presents the factors used in the classification process, provides an overview of typical median designs, identifies the factors that influence the safety and operational performance of median openings, and presents typical combinations of median opening designs used along an arterial. Read the entire page →
From page 34... ... The discussion of each median opening design includes a figure with a diagram of each median opening design and a list of the advantages and disadvantages associated with each design. Conventional Midblock Median Opening A conventional midblock median opening permits vehicles to make U-turns, but does not provide separate channelized roadways for vehicles making U-turns in opposing directions. Read the entire page →
From page 35... ... • Accident rates at midblock median openings are lower than at three- or four-leg median openings. Disadvantages • The absence of left-turn lanes increases potential for rear-end collisions between U-turn vehicles and following through vehicles. Read the entire page →
From page 36... ... Directional median openings are appropriate where U-turn volumes are relatively high, such that U-turn vehicles in opposing directions of travel would otherwise interfere with one another. 36 The directional midblock median opening design is further classified into three subcategories based on the presence of left-turn lanes and/or loons: • Type 2a -- Directional Midblock Median Opening Without Left-Turn Lanes; • Type 2b -- Directional Midblock Median Opening With Left-Turn Lanes; and • Type 2c -- Directional Midblock Median Opening With Left-Turn Lanes and Loons. Read the entire page →
From page 37... ... 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 37 road and the minor road (or driveway) Read the entire page →
From page 38... ... 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. 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. Read the entire page →
From page 39... ... 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) Read the entire page →
From page 40... ... Directional Median Opening for Left Turns from Major Road at Four-Leg Intersection A directional median opening at a four-leg intersection permits vehicles to turn left off the major road onto the minor road and to make U-turn maneuvers on the major road, but does not permit vehicles to turn left from the minor road onto the major road. 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. Read the entire page →
From page 41... ... In Type 7b, the U-turning vehicle exits the major road from the outer lane, loops around to the left, stops clear of the divided highway until a suitable gap in the traffic stream develops, and then makes a normal left turn onto the divided highway. Type 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. Read the entire page →
From page 42... ... Type 3b -- Conventional Median Opening With One Left-Turn Lane at Three-Leg Intersection Advantages • The presence of a left-turn lane in one direction of travel reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles for one direction of travel only on the major road. • The presence of a left-turn lane in the direction of travel mitigates the problem of U-turn vehicles in that direction encroaching on the adjacent lane and interfering with through traffic while waiting for a gap in the opposing traffic. Read the entire page →
From page 43... ... Advantages and disadvantages of median opening type 3c -- conventional median opening with two left-turn lanes at three-leg intersection. Type 3c -- Conventional Median Opening With Two Left-Turn Lanes at Three-Leg Intersection Advantages • 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. Read the entire page →
From page 44... ... The safety and operational performance of a median opening without a left-turn lane is largely dependent on the operating speeds of the vehicles on the roadway. Median openings, and other intersections, can experience large speed 44 differentials between turning vehicles and following through traffic. Read the entire page →
From page 45... ... • Vehicles on the major road can make left turns onto the minor road without having to accommodate minor-road vehicles making left-turn maneuvers through the median opening. • Vehicles on the major road can make U-turn maneuvers in one direction of travel. Read the entire page →
From page 46... ... To reduce the potential for angle collisions, some median opening designs proType 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. Read the entire page →
From page 47... ... • Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the cross street may experience long delays. • There are 32 potential conflict points with both minor-road and majorroad vehicles entering the median opening. Read the entire page →
From page 48... ... Type 5b -- Conventional Median Opening With Left-Turn Lanes at Four-Leg Intersection Advantages • 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. • The presence of left-turn lanes mitigates the problem of U-turn vehicles encroaching on adjacent lanes and interfering with through traffic while waiting for a gap in the opposing traffic. Read the entire page →
From page 49... ... • Accident rates at directional four-leg median openings are lower than at conventional four-leg median openings. Disadvantages • Minor-road vehicles do not have direct left-turn access to the major road. Read the entire page →
From page 50... ... median is required for this design. • U-turn vehicles must make a crossing maneuver, which is not required at conventional or directional midblock median openings. Read the entire page →
From page 51... ... operating speed access density Operational demands major-road volume minor-road volume left-turn volume U-turn volume Physical constraints sight distance right-of-way width building setback Roadway geometrics number of lanes median width Nearby features proximity to driveways proximity to traffic signal proximity to adjacent median openings proximity to unsignalized intersections TABLE 14 Factors that influence the operational and safety performance of median openings Read the entire page →
From page 52... ... 52 RELATIVE SAFETY OF MEDIAN OPENING DESIGNS BASED ON TRAFFIC CONFLICT POINTS The classification of median opening designs provides a framework for comparing the relative safety of each design. The results of the accident data analysis, presented in Chapter 6 of this report, provide information on the relative safety performance of those median opening designs for which sufficient data existed. Read the entire page →
From page 53... ... Chapter 6 of this report presents two examples of a safety comparison between an individual median opening and a combination of median openings accommodating the same turning movements. Combination C1 -- Directional Midblock Median Opening Between Four-Leg Intersections With Directional Median Openings Combination C1 consists of a directional midblock median opening with left-turn lanes located between two four-leg intersections with left-turn lanes and directional median openings Figure 37. Read the entire page →
From page 54... ... 54 Combination C3 -- Directional Midblock Median Opening Between Four-Leg Intersections With Conventional Median Openings Combination C3 includes a directional midblock median opening with left-turn lanes located between two four-leg intersections with left-turn lanes and conventional median openings. This combination is illustrated in Figure 40 and is represented as Combination C3 = Type 5b + Type 2b + Type 5b Combination C3 provides access to through vehicles on the major road to development on the opposite side of the median. Read the entire page →
From page 55... ... Figure 40. Combination C3 -- Directional midblock median opening between four-leg intersections with conventional median openings. Read the entire page →
From page 56... ... Combination C6 -- Conventional Midblock Median Opening Between Four-Leg Intersections Without Median Openings Combination C6 includes a conventional midblock median opening with left-turn lanes between two four-leg intersec56 tions without median openings. This combination is illustrated in Figure 43 and is represented as Combination C6 = Closed Type 5a + Type 1b + Closed Type 5a Combination C6 is similar to Combination C4, except that Combination C6 has a conventional rather than a directional midblock median opening. Read the entire page →
From page 57... ... Combination C8 = Signalized Type 5a + Type 2b + Signalized Type 5a Traffic Signal Traffic Signal Figure 45. Combination C8 -- directional midblock median opening between four-leg signalized intersections with conventional median openings but all left-turning movements prohibited. Read the entire page →
From page 58... ... Major-road vehicles must make left-turn or U-turn movements by proceeding through the intersection, making Uturns at the midblock median opening, and turning right onto the minor road. This combination is appropriate where conventional median openings at signalized four-leg intersections are experiencing safety problems such that the most effective treatment is to reduce the number of conflicts by prohibiting turning movements at the intersections. Read the entire page →

From page 33...

... This classification system identifies how particular median openings function and where they are located relative to other elements of the highway system. This chapter presents the factors used in the classification process, provides an overview of typical median designs, identifies the factors that influence the safety and operational performance of median openings, and presents typical combinations of median opening designs used along an arterial.