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Safety of U-Turns at Unsignalized Median Openings (2004)

Chapter: Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings

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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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Suggested Citation:"Appendix C - Guidelines for the Use, Location, and Design of Unsignalized Median Openings." National Academies of Sciences, Engineering, and Medicine. 2004. Safety of U-Turns at Unsignalized Median Openings. Washington, DC: The National Academies Press. doi: 10.17226/13768.
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C-1 APPENDIX C GUIDELINES FOR THE USE, LOCATION, AND DESIGN OF UNSIGNALIZED MEDIAN OPENINGS This appendix presents guidelines for the use, location, and design of unsignalized median openings. The guidelines include a classification and assessment of typical median opening designs, a discussion of the factors that influence the safety and operational performance of median openings, and a methodology that can be used to compare the expected safety performance of two or more median opening design alternatives. MEDIAN TREATMENTS The selection of median treatment has important bearing on how well a roadway will operate, its safety performance, and the access provided to adjacent developments. The basic options for median treatment include: • No median (undivided roadway) • Continuous two-way left-turn lane (TWLTL) • Nontraversable median NCHRP Report 395, Capacity and Operational Effects of Mid-Block Left-Turn Lanes (15), compares the relative traf- fic operational and safety performance of these three median options. Table C-1 presents a comparison of these three median options, indicating which option is preferred with respect to operational, safety, access, and other factors. Many studies have analyzed the safety benefits of installing TWLTLs or nontraversable medians on undivided highways, and replacing TWLTLs with nontraversable medians. NCHRP Report 420, Impacts of Access Management Techniques (4), presents a summary of the individual studies and compares their results for each of the three median options. Figures C-1 and C-2 present accident rates by median type and total access density (both directions) for urban-suburban and rural road- ways, respectively. Roadways with nontraversable medians consistently have a lower accident rate than undivided road- ways or roadways with TWLTLs. The preceding tables and figures show the reasons why high- way agencies are increasingly using nontraversable medians on arterial highways. Provision of a nontraversable median is likely to result in increased U-turn demand. However, it is unlikely that the increased U-turn demand can be met solely at signalized intersections. Therefore, unsignalized median openings are likely needed to accommodate both U-turn demand and left-turn access, where appropriate. The follow- ing 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 par- ticular median openings function and where they are located relative to other elements of the highway system. This sec- tion presents the factors used in the classification process and provides an overview of typical median designs. Factors Used in Classification of Median Opening Designs The four key factors used to classify or describe the design of a median opening are: • Type of geometry • Degree of access served • Presence of left-turn lanes • Presence of loons The first factor, type of geometry, determines which move- ments are possible at a median opening. Conventional median openings (sometimes referred to as “full median openings”) typically permit all movements, while directional median openings may restrict certain movements. Jughandles are an indirect left-turn treatment that enable drivers to make U-turn and left-turn maneuvers efficiently on divided highways with relatively narrow medians. The second factor, degree of access served, not only deter- mines what movements need to be accommodated at a median opening, but also the number of potential conflict points a median opening will have. For example, a median opening that only serves U-turn maneuvers will have considerably fewer conflicting maneuvers than a median opening at a three- or four-leg intersection, where U-turns use the same roadway as left-turn and crossing maneuvers. Median open- ings can be classified by whether access points are present on neither side, one side, or both sides of the roadway. Access points at median openings may include either intersecting public roads or driveways. The third factor used in the classification is whether or not a median opening has a left-turn lane. Median openings gen- erally operate better when left-turn lanes are present to pro- vide a deceleration and storage area for vehicles before they enter the median. In fact, the AASHTO Green Book specifi- cally encourages the use of left-turn lanes at median open- ings to reduce or eliminate stopping on the through lanes (3).

C-2 TABLE C-1 Comparison of effects of three alternative cross-sections with differing midblock left-turn treatment types (15) Figure C-1. Estimated accident rates by type of median-urban and suburban areas.

The final factor in classification of median openings is whether or not a median opening is accompanied by a loon. A loon is an expanded paved apron on the shoulder opposite a median crossover. The purpose of loons is to provide addi- tional space for larger vehicles (particularly trucks) to nego- tiate turns, and thus, to allow the installation of conventional or directional median openings along narrow medians. The provision of loons to serve U-turns by large vehicles is a new technique that formalizes past use of paved shoulders for the same purpose. Initial results by highway agencies that have used loons appear promising (55, 56). Based on the four factors discussed above, median open- ings can be classified based on their design characteristics as follows: • Type of geometry (traffic movements permitted) – conventional (all movements permitted) – directional • Degree of access served – U-turn only (midblock median opening) – access on one side (at three-leg intersection) – access on two sides (at four-leg intersection) • Presence of left-turn lane – no left-turn lane present – left-turn lane present • Presence of loon – no loon present – loon present C-3 Overview of Typical Median Opening Designs Using the first two classification factors (geometry type and degree of access served), typical median openings can be classified into the following six categories: 1. Conventional Midblock Median Opening 2. Directional Midblock Median Opening 3. Conventional Median Opening at Three-Leg Intersection 4. Directional Median Opening at Three-Leg Intersection 5. 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. With these subdivisions, there are a total of fifteen typical median opening designs. The following discussion presents each of the six categories of median openings and the specific designs used for those open- ings. 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 Figure C-2. Estimated accident rates by type of median-rural areas (4).

roadways for vehicles making U-turns in opposing directions. Median openings at midblock locations are appropriate on arterials where providing for U-turn maneuvers between inter- sections may improve operations at intersections by reducing the U-turn volumes at those intersections or reducing the amount of out-of-direction travel for vehicles trying to reach a destination without direct left-turn access. Conventional median openings are appropriate where U-turn volumes are relatively low, such that U-turn vehicles in opposing direc- tions of travel create minimal interference with one another. The conventional midblock median opening design is fur- ther classified into three subcategories based on the presence of left-turn lanes and/or loons: Type 1a—Conventional Midblock Median Opening With- out 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. The presence of left-turn lanes in Types 1b and 1c reduces the potential for rear-end collisions between U-turn vehicles and following through vehicles. The presence of loons in Type 1c provides a widening in the pavement to accommodate U-turn movements by larger vehicles, such as emergency vehicles and trucks. Directional Midblock Median Opening A directional midblock median opening permits vehicles to make U-turns and provides separate channelized roadways for vehicles making U-turns in opposite directions. Thus, oppos- ing U-turn vehicles will not overlap. Median openings at mid- Type 1a—Conventional Midblock Median Opening Without Left-Turn Lanes Advantages • Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. • Since only major-road traffic is involved, delays to vehicles making U-turns will be less than where an intersection is present. • If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only four conflict points, which is less than at a three-leg or four-leg intersection. • 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. Disadvantages • The absence of left-turn lanes increases potential for rear-end collisions between U-turn vehicles and following through vehicles. • U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. • With no directional island, opposing U-turn vehicles may have to overlap. Figure C-3. Advantages and disadvantages of median opening type 1a— conventional midblock median opening without left-turn lanes.

block locations are appropriate on arterials where providing for U-turn maneuvers between intersections may improve operations at intersections by reducing the U-turn volumes at those intersections or reducing the amount of out-of-direction travel for vehicles trying to reach a destination without direct left-turn access. 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. 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 C-5 Type 2c—Directional Midblock Median Opening With Left-Turn Lanes and Loons Figures C-6 through C-8 illustrate these three median open- ing designs and their advantages and disadvantages. The pres- ence of left-turn lanes in Types 2b and 2c reduces the potential for rear-end collisions between U-turn vehicles and following through vehicles. The presence of loons in Type 2c provides a widening in the pavement to accommodate U-turn movements by larger vehicles, such as emergency vehicles and trucks. Conventional Median Opening at Three-Leg Intersection A conventional median opening at a three-leg intersection permits vehicles on the major road to make U-turn move- ments on the major road and left- or right-turning movements Type 1b—Conventional Midblock Median Opening With Left-Turn Lanes Advantages • The presence of left-turn lanes reduces potential for rear-end collisions between U-turn vehicles and following through vehicles. • 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. • Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. • Since only major-road traffic is involved, delays to vehicles making U- turns will be less than where an intersection is present. • If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only four conflict points, which is less than at a three-leg or four-leg intersection. • 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. Disadvantages • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. • With no directional island, opposing U-turn vehicles may have to overlap. Figure C-4. Advantages and disadvantages of median opening type 1b—conventional midblock median opening with left-turn lanes.

onto the minor road. Vehicles on the minor road may make left or right turns onto the major road. No separate channel- ized roadways are provided for vehicles making U-turns in opposing directions. Thus, U-turn vehicles may overlap with opposing U-turn or left-turn vehicles. 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) and where U-turn volumes are relatively low, such that U-turn vehicles in opposing direc- tions of travel create minimal interference with one another. The conventional median opening design at a three-leg intersection is further classified into four subcategories based on the presence of a left-turn lane and/or loon: C-6 Type 3a—Conventional Median Opening Without Left- Turn Lanes at Three-Leg Intersection Type 3b—Conventional Median Opening With One Left- Turn Lane at Three-Leg Intersection Type 3c—Conventional Median Opening With Two Left- Turn Lanes at Three-Leg Intersection Type 3d—Conventional Median Opening With Left-Turn Lanes and Loons at Three-Leg Intersection Figures C-9 through C-12 illustrate these four median opening designs and their advantages and disadvantages. The presence of left-turn lanes in Types 3b, 3c, and 3d reduces the potential for rear-end collisions between U-turn vehicles and following through vehicles. The presence of loons in Type 3d provides a widening in the pavement to accommo- date U-turn movements by larger vehicles, such as emer- gency vehicles and trucks. Type 1c—Conventional Midblock Median Opening With Left-Turn Lanes and Loons Advantages • Widening on the far side of the turn makes it possible to make a U-turn without stopping or backing and reduces interference between U-turn and through traffic, particularly for large vehicles. • Without performing major reconstruction, additional space can be provided to facilitate the larger turning path of commercial vehicles along narrow medians. • The presence of left-turn lanes reduces potential for rear-end collisions between U-turn vehicles and following through vehicles. • 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. • Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. • Since only major-road traffic is involved, delays to vehicles making U- turns will be less than where an intersection is present. • If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. • 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. Disadvantages • U-turn vehicles entering the through lanes may delay full-speed through traffic. • With no directional island, opposing U-turn vehicles may have to overlap. • The presence of loons may make snow removal and other maintenance work more difficult. Figure C-5. Advantages and disadvantages of median opening type 1c—conventional midblock median opening with left-turn lanes and loons.

Directional Median Opening at Three-Leg Intersection There are two types of directional median openings at three- leg intersections: Type 4a—Directional Median Opening for Left Turns from Major Road at Three-Leg Intersection Type 4b—Direction Median Opening for Left Turns onto Major Road at Three-Leg Intersection The first type, designated as Type 4a, 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. The second type, designated as Type 4b, permits vehicles to turn left or right from the minor road onto the major road and vehicles on the major road to make U-turn maneuvers, but does not permit vehicles to turn left off the major road onto C-7 the minor road. 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 traf- fic operational effects. Directional median openings are appropriate where U-turn or left-turn volumes are relatively high, such that a conven- tional median opening would experience considerable inter- ference between vehicles entering the median opening. Direc- tional median openings are also appropriate where there is a disproportionately high left-turn demand from either the major road or the minor road and, therefore, either Type 4a or Type 4b would accommodate the needs of the intersection. Directional median openings are desirable where an inter- section is going to be signalized, since it only impacts major- road traffic in one direction and effective two-direction sig- nal coordination can be maintained (5). Figures C-13 through C-14 illustrate these two median opening designs and their advantages and disadvantages. Type 2a—Directional Midblock Median Opening Without Left-Turn Lanes Advantages • This design prevents overlapping U-turns. • Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. • Since only major-road traffic is involved, delays to vehicles making U- turns will be less than where an intersection is present. • If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only four conflict points, which is less than at a three-leg or four-leg intersection. • 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. Disadvantages • The absence of left-turn lanes increases potential for rear-end collisions between U-turn vehicles and following through vehicles. • U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. Figure C-6. Advantages and disadvantages of median opening type 2a—directional midblock median opening without left-turn lanes.

Conventional Median Opening at Four-Leg Intersection A conventional median opening at a four-leg intersection permits vehicles on the major road to make U-turn move- ments on the major road and left- or right-turning movements onto the minor road. Vehicles on the minor road may make left or right turns onto the major road. No separate channel- ized roadways are provided for vehicles making U-turns in opposing directions. Thus, U-turn vehicles may overlap with opposing U-turn or left-turn vehicles. 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) and where U-turn volumes are rel- C-8 atively low, such that U-turn vehicles in opposing directions of travel create minimal interference with one another. The conventional median opening design at a four-leg inter- section is further classified into two subcategories based on the presence of left-turn lanes: Type 5a—Conventional Median Opening Without Left- Turn Lanes at Four-Leg Intersection Type 5b—Conventional Median Opening With Left-Turn Lanes at Four-Leg Intersection Figures C-15 and C-16 illustrate these two median opening designs and their advantages and disadvantages. The presence of left-turn lanes in Type 5b reduces the potential for rear- end collisions between U-turn vehicles and following through vehicles. Type 2b—Directional Midblock Median Opening With Left-Turn Lanes Advantages • The presence of left-turn lanes reduces potential for rear-end collisions between U-turn vehicles and following through vehicles. • 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. • This design prevents overlapping U-turns. • Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. • Since only major-road traffic is involved, delays to vehicles making U- turns will be less than where an intersection is present. • If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only four conflict points, which is less than at a three-leg or four-leg intersection. • 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. Disadvantages • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. Figure C-7. Advantages and disadvantages of median opening type 2b—directional midblock median opening with left-turn lanes.

Directional Median Opening for Left Turns from Major Road at Four-Leg Intersection A directional median opening at a four-leg intersection per- mits 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. Directional median openings are appro- priate 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 left- turn demand from the major road. Directional median open- ings are desirable where an intersection is going to be signal- ized, since it only impacts major-road traffic in one direction C-9 and effective two-direction signal coordination can be main- tained (NCHRP Report 348). This median opening design is represented in the classification as: Type 6a—Directional Median Opening for Left Turns from Major Road at Four-Leg Intersection Figure C-17 illustrates this median opening design and its advantages and disadvantages. LOCATION AND DESIGN GUIDELINES This section presents guidelines on the location and design of median openings based on the following factors: • Major- and minor-road volumes • Left-turn and U-turn volumes • Median width • Median opening length Type 2c—Directional Midblock Median Opening With Left-Turn Lanes and Loons Advantages • Widening on the far side of the turn makes it possible to make a U-turn without stopping or backing and reduces interference between U-turn and through traffic, particularly for large vehicles. • Without performing major reconstruction, additional space can be provided to facilitate the larger turning path of commercial vehicles along narrow medians. • The presence of left-turn lanes reduces potential for rear-end collisions between U-turn vehicles and following through vehicles. • 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. • This design prevents overlapping U-turns. • Midblock access is provided for vehicles to (a) make a U-turn and (b) reach driveways on the opposite side of the street. • Since only major-road traffic is involved, delays to vehicles making U-turns will be less than where an intersection is present. • If U-turn traffic would otherwise proceed downstream to an intersection with a median opening, then this treatment should reduce VMT. • 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. Disadvantages • The presence of loons may make snow removal and other maintenance work more difficult. Figure C-8. Advantages and disadvantages of median opening Type 2c—directional midblock median opening with left-turn lanes and loons.

• Minimum spacing between median openings • Sight distance • Loons to assist vehicles in completing U-turn maneuvers • Median opening types Major- and Minor-Road Volumes Unsignalized median openings may be used for a broad range of major- and minor-road traffic volumes. However, if the major- and minor-road volumes exceed the traffic vol- umes given in the MUTCD signalization warrants, signal- ization of the median opening should be considered. No safety prediction models relating median opening acci- dent frequencies to major- and minor-road volumes are avail- C-10 able. Safety prediction models for divided highway intersec- tions that consider major- and minor-road traffic volumes are currently being developed for use in the Highway Safety Man- ual. Once appropriate safety prediction models are developed, they should be used in comparing alternative median opening designs. While the full major- and minor-road traffic volumes cannot currently be considered in the location and design of median openings, consideration can be given to median open- ing volumes (U-turns and left turns), 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 rang- 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. • Vehicles on the major road can make left and right turns onto the minor road and make U-turns on the major road. • Delays will be less than at a four-leg intersection. • No additional VMT is incurred by vehicles making left-, right-, or U-turning maneuvers. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only 11 conflict points, which is less than at a four-leg intersection. • Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages • The absence of left-turn lanes increases potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles. • U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. • With no directional island, opposing U-turn vehicles may have to overlap. • Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where six-lane or eight-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. • Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. Figure C-9. Advantages and disadvantages of median opening type 3a—conventional median opening without left-turn lanes at three-leg intersection.

ing from 2 to 977 veh/day, representing from 0.01 to 3.2 per- cent of the major-road traffic volumes at those locations. At median openings in rural areas, U-turn volumes were found to range from 88 to 374 veh/day, representing 0.41 to 1.36 percent of the major-road traffic volumes at those locations. The available data are not sufficient to develop satisfactory regression relationships to relate median opening accident C-11 frequency to median opening volumes. It is also not possible to separately account for the effects of U-turn and left-turn volumes on safety at median openings. However, Table C-2 presents the accident rates per median opening traffic move- ment 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. Computational proce- 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. • Vehicles on the minor road can make left and right turns onto the major road. • Vehicles on the major road can make left and right turns onto the minor road and make U-turns on the major road. • Delays will be less than at a four-leg intersection. • No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only 11 conflict points, which is less than at a four-leg intersection. • Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages • U-turn vehicles in the direction of travel without a left-turn lane forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. • Vehicles on the major road in the direction of travel without a left-turn lane must initiate U-turn maneuvers from the through lane, which may delay through vehicles and create the potential for rear-end collisions. • With no directional island, opposing U-turn vehicles may have to overlap. • Minimum 6- to 8-second gaps are needed to cross traffic, possibly even 10 sec where six-lane or eight-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. • Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. Figure C-10. Advantages and disadvantages of median opening type 3b—conventional median opening with one left-turn lane at three-leg intersection.

dures for using the average accident rates in Table C-2 in comparing alternative median opening arrangements are pre- sented later in these guidelines. The reason that the effects of U-turn and left-turn volumes on median opening accident frequency cannot be separated is that review of accident data for median openings have found that accident report data do not distinguish clearly between accidents involving U-turn maneuvers and those involving left-turn maneuvers. In particular, at some median openings where U-turn maneuvers can be made but no left-turn maneu- vers are feasible, investigating officers classified a substantial C-12 proportion of the accidents involving turning movements through the median as related to left-turn maneuvers. Left-Turn Lanes Vehicles turning left from a multilane highway may pose safety and operational problems at median openings. They not only increase conflicts with and delays to other vehicles, but also pose a major safety problem with the large speed dif- ferential 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. • 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. • Vehicles on the minor road can make both left and right turns onto the major road. • Vehicles on the major road can make left and right turns onto the minor road and make U-turns on the major road. • Delays will be less than at a four-leg intersection. • No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • There are only 11 conflict points, which is less than at a four-leg intersection. • Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. Advantages • With no directional island, opposing U-turn vehicles may have to overlap. • Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. • Where peak-hour traffic exceeds critical volumes, vehicles entering the major road from the minor road may experience long delays. Figure C-11. Advantages and disadvantages of median opening type 3c—conventional median opening with two left-turn lanes at three-leg intersection.

In a recent FHWA study, a before-after evaluation of the safety effects of providing left-turn lanes for at-grade inter- sections was conducted. The research concluded that provid- ing left-turn lanes is effective in improving safety at unsignal- ized intersections in both rural and urban areas. Specifically, at urban unsignalized intersections, the research found that C-13 installation of a left-turn lane on one approach would be expected to reduce accidents by 27 percent for four-leg inter- sections and by 33 percent for three-leg intersections. Left-turn lanes are often installed at median openings to accommodate high left-turning volumes. Harmelink (67) pro- vides volume warrants and design charts for left-turn lanes at Type 3d—Conventional Median Opening With Left-Turn Lanes and Loons at Three-Leg Intersection Advantages • Widening on the far side of the turn makes it possible to make a U-turn without stopping or backing and reduces interference between U-turn and through traffic, particularly for large vehicles. • Without performing major reconstruction, additional space can be provided to facilitate the larger turning path of commercial vehicles along narrow medians. • 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. • Vehicles on the minor road can make left and right turns onto the major road. • Vehicles on the major road can make left and right turns onto the cross street and make U-turns on the major road. • Delays will be less than at a four-leg intersection. • No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • Accident rates at three-leg intersections are lower than at four-leg intersections. Disadvantages • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Narrow medians may not provide enough space for larger vehicles to negotiate a U-turn. • With no directional island, opposing U-turn vehicles may have to overlap. • Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. • 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. Figure C-12. Advantages and disadvantages of median opening type 3d—conventional median opening with left-turn lanes and loons at three- leg intersection.

unsignalized intersections on four-lane highways. Many high- way agencies, however, have adopted a policy of providing left-turn lanes at virtually all median openings at divided highways. Median Width The safety and operational effects of median width at sig- nalized and unsignalized intersections on divided highways were evaluated extensively in NCHRP Report 375, Median Intersection Design (8), and the design policy recommenda- tions of that report have been incorporated in the AASHTO Green Book (3). Guidelines for selecting median widths at unsignalized intersections on divided highways, as recom- mended in NCHRP Report 375 and the Green Book, are pre- sented below. C-14 Rural Unsignalized Intersections • Rural unsignalized intersections should have medians that are as wide as practical, as long as the median is not so wide that approaching vehicles on the crossroad can- not see both roadways of the divided highway. • Where the AASHTO passenger car is used as the design vehicle, a minimum median width of 8 m (25 ft) is recommended. • Where a large truck is used as the design vehicle, a median width of 21 to 31 m (70 to 100 ft) generally should be selected. Suburban Unsignalized Intersections • Median widths at suburban unsignalized intersections generally should be as narrow as possible while provid- Type 4a—Directional Median Opening for Left Turns From Major Road at Three-Leg Intersection Advantages • The presence of a left-turn lane reduces potential for rear-end collisions between left-turn or U-turn vehicles and following through vehicles on the major road. • The presence of a left-turn lane 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. • Vehicles on the major road have direct left-turn access to the minor road. • Vehicles on the major road can make U-turn maneuvers in one direction of travel. • Right-turn maneuvers are unaffected by this treatment. • For those maneuvers permitted, delays will be less than at a conventional median opening at a three-leg intersection. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • 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. Disadvantages • Minor road vehicles do not have direct left-turn access to the major road. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Additional VMT will be incurred by vehicles whose drivers desire to turn left from the minor road onto the major road. Figure C-13. Advantages and disadvantages of median opening type 4a—directional median opening for left turns from major road at three- leg intersection.

ing sufficient space in the median for the appropriate left-turn treatment. • Median widths between 4.2 and 7.2 (14 and 24 ft) will accommodate left-turn lanes, but are not wide enough to store a crossing or turning vehicle in the median. • Medians wider than 7.6 m (25 ft) may be used, but cross- road vehicles making turning and crossing maneuvers may stop on the median roadway. • Median widths of more than 15 m (50 ft) generally should be avoided at suburban, unsignalized intersections. The design vehicle for choosing the median width at a divided highway intersection is generally based on the vehi- cle mix for vehicles making turning and crossing maneuvers. However, at unsignalized median openings with substantial U-turn volumes, the vehicle mix for U-turn maneuvers should be a major consideration in selecting the design vehicle for C-15 determining median opening width. Figure C-18 illustrates the AASHTO Green Book criteria for determining the median and roadway widths needed to provide for U-turns at unsignal- ized median openings. Where the full median width and road- way width needed to accommodate U-turn maneuvers can- not be provided, consideration should be given to inclusion of a loon in the design (see below). Median Opening Length NCHRP Report 375 (8) states that median opening lengths at rural divided highway intersections generally should be kept to the minimum possible. Increases in median opening length were found to be correlated with higher rates of unde- sirable driving behavior. 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. • Right-turn maneuvers are unaffected by this treatment. • Delays will be less than at a conventional median opening at a three-leg intersection. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • 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. Disadvantages • Vehicles on the major road do not have direct left-turn access to the cross street. • The absence of a left-turn lane on the major road increases potential for rear- end collisions between U-turn vehicles and following through vehicles. • U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Additional VMT will be incurred by vehicles whose drivers desire to turn left from the major road onto the minor road. Figure C-14. Advantages and disadvantages of median opening type 4b— directional median opening for left turns onto major road at three-leg intersection.

greater than necessary (as noted above), but there does not appear to be any reason that the median opening should not be as long as necessary. Minimum Spacing Between Median Openings Guidelines for the minimum spacing between median openings are based on recommendations of NCHRP Report 420 (4) and the results of the highway agency survey conducted as part of this research. They are: • Median opening spacing for rural areas typically ranges from 150 to 805 m (500 to 2,640 ft); a minimum median opening spacing of 150 m (500 ft) is recommended in rural areas. Typically, median opening spacing substan- tially longer than 150 m (500 ft) is appropriate, unless C-16 two public road intersections or major driveways are located relatively close together. • Median opening spacing for urban areas typically ranges from 90 to 805 m (300 to 2,640 ft); a minimum median opening spacing of 90 m (300 ft) is recom- mended in urban areas. Whenever practical, median opening spacing greater than 90 m (300 ft) should be used in urban areas. Sight Distance Intersection sight distance (ISD) is an important design and operational consideration at all intersections, but is espe- cially important at divided highway intersections, including unsignalized median openings, where the presence of the median may increase the ISD needs or provide a location for potential sight obstructions that reduce the ISD. U-turn maneu- Type 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. • No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. Disadvantages • The absence of left-turn lanes increases potential for rear-end collisions between U-turn vehicles and following through vehicles. • U-turn vehicles forced to stop in the median opening may encroach on adjacent lanes and interfere with through traffic. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • With no directional island, opposing U-turn vehicles may have to overlap. • Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. • 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 major- road vehicles entering the median opening. • Accident rates at four-leg intersections are higher than at three-leg intersections. Figure C-15. Advantages and disadvantages of median opening type 5a—conventional median opening without left-turn lanes at four-leg intersection.

vers should not be encouraged at locations with limited sight distance. Furthermore, sight distance is an important issue in determining locations where U-turns by larger vehicles should be permitted/encouraged. NCHRP Report 383 (57) presents revised ISD models that have been adopted by AASHTO and incorporated into the 2001 Green Book (3). The following ISD cases are applica- ble to unsignalized median openings: • Case B1 (left turns from the minor road) • Case B3 (crossing maneuvers) • Case F (left turns from the major road) Each ISD case is described in more detail below. C-17 Left Turns from the Minor Road (Case B1) Case B1 involves a situation in which a vehicle is stopped on the minor road awaiting an opportunity to complete a left- turn maneuver by clearing traffic approaching from the left and then enters the traffic stream approaching from the right. Green Book Exhibit 9-55 presents design ISD criteria for Case B1. At divided-highway intersections, sight distance design for left turns may need to consider multiple design vehicles. If the design vehicle used to determine sight distance for a divided-highway intersection is larger than a passenger car, then sight distance for left turns will need to be checked for that selected design vehicle. The Green Book includes guid- 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. • 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 can make left and right turns onto the minor road and make U-turns on the major road. • No additional VMT is incurred by vehicles making left-, right-, or U- turning maneuvers. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. Disadvantages • U-turn vehicles entering the through lanes may delay full-speed through traffic. • With no directional island, opposing U-turn vehicles may have to overlap. • Minimum 6- to 8-sec gaps are needed to cross traffic, possibly even 10 sec where 6-lane or 8-lane arterials must be crossed. These gaps will be limited during busy travel period on heavily used arterials. • 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 cross-street and major-road vehicles entering the median opening. • Accident rates at four-leg intersections are higher than at three-leg intersections. Figure C-16. Advantages and disadvantages of median opening type 5b—conventional median opening with left-turn lanes at four- leg intersection.

ance for median widths large enough to store the design vehicle and for median widths not large enough to store the design vehicle. Crossing Maneuvers (Case B3) Case B3 involves a situation in which a vehicle is stopped on the minor road awaiting an opportunity to cross the major road by clearing traffic approaching from the left and from the right. Green Book Exhibit 9-58 presents design ISD cri- teria for Case B3. In most cases, the sight distance provided in Case B1 will provide more than adequate sight distance for minor-road vehicles to cross the major road. However, in the following situations, it is advisable to check the availability of sight distance for crossing maneuvers: C-18 • where left and/or right turns are not permitted from a par- ticular approach and the crossing maneuver is the only legal maneuver; • where the crossing vehicle would cross the equivalent width of more than six lanes; or • where substantial volumes of heavy vehicles cross the highway and steep grades that might slow the vehicles while its back portion is still in the intersection are pre- sent on the departure roadway on the far side of the intersection. Left Turns from the Major Road (Case F) Case F involves a situation in which a vehicle is stopped on the major road awaiting an opportunity to complete a left- Type 6a—Directional Median Opening for Left Turns From Major Road 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. • Vehicles on the major road have direct left-turn access to the minor road. • Vehicles on the major road in both directions of travel can make U-turn maneuvers. • Right-turn maneuvers are unaffected by this treatment. • Delays will be less than at a conventional median opening at a four-leg intersection. • Since vehicles making a U-turn only need to enter, but not cross, the opposing roadway, a minimum gap of only 4 to 6 sec will be needed. • 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. Disadvantages • Minor-road vehicles do not have direct left-turn access to the major road. • U-turn vehicles entering the through lanes may delay full-speed through traffic. • Accident rates at four-leg intersections are higher than at three-leg intersections. • No direct crossing maneuver on minor road. • Additional VMT will be incurred by vehicles whose drivers desire to make crossing maneuvers on the minor road and left-turn maneuvers from the minor road to the major road. Figure C-17. Advantages and disadvantages of median opening type 6a—directional median opening for left turns from major road at four-leg intersection.

turn maneuver by waiting for an appropriate gap in opposing traffic to complete their turn. Green Book Exhibit 9-67 pre- sents design ISD criteria for Case F. On divided highways, while the geometry of the roadway may provide sufficient sight distance for left turns from the major road, some intersections may experience additional sight-distance concerns, such as: (1) sight obstructions in the median or (2) opposing left-turn vehicles obstructing the view of a left-turning vehicle. The sight restrictions created by opposing left-turn vehicles on divided highways can be minimized by the use of parallel and tapered offset left-turn lanes, as shown in Green Book Exhibit 9-98. Loons to Assist Vehicles in Completing U-turn Maneuvers A common problem associated with accommodating U-turn maneuvers at unsignalized median openings is the difficulty of larger vehicles to negotiate U-turns along cross-sections with narrow medians. This situation often affects the operation and safety of commercial vehicles that typically require more space in order to perform a U-turn maneuver. One possible solution to this problem is the construction of a loon. Loons C-19 are defined as expanded paved aprons opposite a median crossover. Their purpose is to provide additional space to facil- itate the larger turning path of commercial vehicles along nar- row medians. Figure C-19 presents a typical loon design. Where a large truck is used as the design vehicle for a median opening and a median width of 21 to 31 m (70 to 100 ft) can- not be provided, consideration should be given to provid- ing a loon. Several unsignalized median openings with loons were evaluated as part of this research. No specific problems related to loon operations were noted at these sites. 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 open- ings 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. At the same time, there are not sufficient data to determine whether the provision of loons provides safety benefits. Based on a study by Sisiopiku and Aylsworth-Bonzelet (55, 56), Table C-3 presents recommended loon widths for TABLE C-2 Median opening accident rate by median opening type Median opening type Number of median openings Total median opening accident frequencya (for entire study period) Median opening movements (106 turns during entire study period) Median opening accident rate (accidents per 106 turning vehicles) URBAN ARTERIAL CORRIDORS Midblock 1a 7 1 –b –b 2b 20 4 17.20 0.23 2c 10 5 13.42 0.37 Three-leg 3a 11 9 2.23 4.04 3b 19 32 13.04 2.46 3c 2 10 1.20 8.35 4a 4 7 4.87 1.44 Four-leg 5a 8 26 11.16 2.33 5b 17 76 22.77 3.34 6a 5 42 16.36 2.57 RURAL ARTERIAL CORRIDORS Midblock 1a 7 3 0.96 3.13 Three-leg 3a 4 4 4.65 0.86 Four-leg 5a 1 4 1.41 2.84 a The duration of the study period was generally five years. However, only four years of accident and exposure data were available for sites in New Jersey, and six years of accident and exposure data were available for sites in New York. b Data too limited to be meaningful.

C-20 Figure C-18. AASHTO minimum median widths to accommodate U-turns (3). Figure C-19. Typical loon design at a directional median opening (55, 56).

four-lane divided roadways. Usually, loons are not needed on six-lane divided highways; usually a paved shoulder pro- vides all the additional width needed for U-turns by large trucks. Median Opening Types The four key factors used to classify or describe the design of a median opening are: • Type of geometry—determines which movements are possible at a median opening. Conventional median openings typically permit all movements, while direc- tional median openings may restrict certain movements. • Degree of access served—determines what movements need to be accommodated at a median opening and the number of potential conflict points a median opening will have. Access points at median openings may include either intersecting public roads or driveways. • Presence of left-turn lanes—indicates whether or not a median opening has a left-turn lane. • Presence of loons—indicates whether or not a median opening is accompanied by a loon. Based on these four factors, median openings can be clas- sified based on their design characteristics as follows: • Type of geometry (traffic movements permitted) – conventional (all movements permitted) – directional • Degree of access served – U-turn only (midblock median opening) – access on one side (at three-leg intersection) – access on two sides (at four-leg intersection) • Presence of left-turn lane – no left-turn lane present – left-turn lane present • Presence of loon – no loon present – loon present Drawings illustrating each of the median opening types, along with their advantages and disadvantages of each, are C-21 presented in the preceding section of these guidelines (see Figures C-3 through C-17). METHODOLOGY FOR COMPARING THE EXPECTED SAFETY PERFORMANCE OF MEDIAN OPENING DESIGN ALTERNATIVES This section presents a methodology for comparing the expected safety performance of median opening design alter- natives. 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 per- formance 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 open- ing design alternative Step 5: Compare accident rates for each median open- ing 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. In Step 1, each of the possible median opening design alternatives are identified. Note that an alternative may be an individual median opening design or may include a combination of median opening designs. For example, at a three-leg intersection, a highway agency may be considering the following alternatives: • conventional median opening • directional median opening Type of design vehicle P SU BUS WB-12 WB-15 WB-18 Length of design vehicle (m) 5.7 9.0 12.0 15.0 16.5 19.5 Median width (m) Width of loon (m) 0 5 15 15 15 18 18 5 0 10 10 10 13 13 10 0 5 5 5 8 8 15 0 0 0 0 3 3 20 0 0 0 0 0 0 NOTE: Loon width equal to 0 indicates that the standard shoulder width is sufficient. TABLE C-3 Recommended loon widths for four-lane divided roadways (55, 56)

• directional median opening in combination with a directional midblock median opening located near the intersection. Alternative 1: ____________________________________ Alternative 2: ____________________________________ Alternative 3: ____________________________________ Step 2: Enter all turning volumes Turning volumes—actual or estimated—for all possible turning movements must be provided. Figure C-20 illustrates all possible turning volumes at a four-leg intersection. If the median opening design alternative under consideration is a three-leg or midblock median opening, enter a zero for those turning volumes that do not apply. Some median openings, such as a three-leg median open- ing or a directional median opening, will include only a por- tion of these turning volumes since not all turning move- ments are accommodated. Enter all turning volumes for the median opening design alternatives considered: Volume A (Major-road U-turns in Direction 1) = ___ (veh/day) Volume B (Major-road U-turns in Direction 2) = ___ (veh/day) Volume C (Major-road left turns in Direction 1) = ___ (veh/day) Volume D (Major-road left turns in Direction 2) = ___ (veh/day) C-22 Volume E (Minor-road left turns in Direction 3) = ___ (veh/day) Volume F (Minor-road left turns in Direction 4) = ___ (veh/day) Volume G (Minor-road throughs in Direction 3) = ___ (veh/day) Volume H (Minor-road throughs in Direction 4) = ___ (veh/day) Total turning volume = ___ (veh/day) 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 non- intersection accident rate for the major road is needed. This may be obtained from actual accident data or may be esti- mated based on historical accident experience or engineering judgment. In the absence of any reliable site-specific esti- mate, NCHRP Report 282 (22) suggests 2.90 accidents per million vehicle-miles is an average non-intersection accident rate for all four-lane divided suburban highways. Non-intersection accident rate for the major road = _________ (acc/MVM) Step 4: Calculate the accident rate for each median opening design alternative The accident rate for each median opening design alterna- tive consists of at least one of the following components: Direction 2D ir e c ti o n 4 D ir e c ti o n 3 Direction 1 A C D B GE FH Figure C-20. Turning movements at a four-leg intersection.

• Accident rate for each individual median opening—For median opening design alternatives that consist of a sin- gle median opening, this is the only accident rate that needs to be computed. For median opening design alter- natives that consist of a combination of median open- ings, this accident rate must be computed for each indi- vidual median opening that makes up the combination. • Accident rate for exposure due to additional travel dis- tance—For combinations of median openings, vehicles often travel an additional distance along the major road to complete their turning maneuver. This additional travel distance must be accounted for when computing the estimated safety performance of the combination of median openings. Therefore, the accident rate for the exposure due additional travel distance must be com- puted as well. The equations for calculating each accident rate are pre- sented below. Each equation utilizes Table C-4, which pre- sents median opening accident rates by geometry type (con- ventional vs. directional) and number of intersection legs (midblock vs. three-leg vs. four-leg). Step 4a: Calculate the accident rate for each individual median opening The accident rate for each median opening can be esti- mated by the following equation: where: AR = accident rate from Table C-2 TV = sum of all turning volumes through median opening For combinations of median openings, repeat this step for each individual median opening. Step 4b: Calculate the accident rate for exposure due to additional travel distance The accident rate for the exposure related to additional travel distance can be estimated by the following equation: Accident Rate acc/yr( ) = [ ] × [ ] ×AR acc veh TV veh day days yr10 365 6 C-23 where: ARmr = average non-intersection accident rate for major road TV = sum of all turning volumes traveling additional distance d = distance between midblock median opening and intersection (mi) 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 indi- vidual 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. Alternative 1: ____________________________________ Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident rate (due to exposure) = ___ (acc/yr) Total accident rate = ___ (acc/yr) Alternative 2: ____________________________________ Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident rate (due to exposure) = ___ (acc/yr) Total accident rate = ___ (acc/yr) Alternative 3: ____________________________________ Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident Rate acc/yr( ) = [ ] × ×( ) × [ ] × AR acc veh mi d TV veh day days yr mr 10 2 365 6 - TABLE C-4 Median opening accident rates by geometry type and number of intersection legs at urban sites

Accident rate (individual median opening) = ___ (acc/yr) Accident rate (due to exposure) = ___ (acc/yr) Total accident rate = ___ (acc/yr) Step 5: Compare accident rates for each median opening design alternative The final step in the methodology is to compare the accident rates for each median opening design alternative. If one alter- native has a substantially lower accident rate than the other alternative(s), that alternative may be a preferable median opening design from a safety standpoint. However, the alter- natives should also be compared from an operational stand- point. For example, if one alternative accommodates fewer turning movements than another alternative, selecting that alternative may have a negative impact on the traffic opera- tional performance of the arterial corridor. Example 1: Comparison of the safety performance of conventional and directional median open- ings at three-leg intersections Step 1: Select median opening design alternatives Alternative 1: Conventional median opening at a three- leg intersection Alternative 2: Directional median opening at a three-leg intersection in combination with a direc- tional midblock median opening Figure C-21 illustrates Alternatives 1 and 2. In Alterna- tive 2, the directional median opening at the three-leg inter- section accommodates vehicles making left-turn maneu- C-24 vers from the major road onto the minor road and vehicles making U-turns in Direction 2 of the major road. Left-turn vehicles on the minor road and U-turn vehicles in Direc- tion 1 of the major road must use the directional midblock median opening to complete their turning maneuvers. Step 2: Enter all turning volumes Enter all turning volumes for the median opening design alternatives considered: Volume A (Major-road U-turns in Direction 1) = 25 veh/day Volume B (Major-road U-turns in Direction 2) = 25 veh/day Volume C (Major-road left turns in Direction 1) = N/A Volume D (Major-road left turns in Direction 2) = 225 veh/day Volume E (Minor-road left turns in Direction 3) = 225 veh/day Volume F (Minor-road left turns in Direction 4) = N/A Volume G (Minor-road throughs in Direction 3) = N/A Volume H (Minor-road throughs in Direction 4) = N/A Total turning volume = 500 veh/day Step 3: Enter the average non-intersection accident rate for the major road Non-intersection accident rate for the major road = 2.90 acc/MVM (from NCHRP Report 282) a) Conventional Median Opening at Three-Leg Intersection (Type 3b) b) Directional Median Opening For Left Turns From Major Road at Three-Leg Intersection With Directional Midblock Median Opening (Type 4a + Type 2b) Figure C-21. Conventional and directional median openings at three-leg intersection.

Step 4: Calculate the accident rate for each median opening design alternative For Alternative 1, only the accident rate for a conventional median opening at a three-leg intersection needs to be computed. For Alternative 2, the accident rates for a directional median opening at a three-leg intersection, a directional midblock median opening, and the exposure due to additional travel distance need to be computed. Step 4a: Calculate the accident rate for each individ- ual median opening Conventional three-leg: The accident rate for a conventional median opening at a three-leg intersection is computed as follows: From Table C-2, the median opening accident rate for a con- ventional three-leg median opening is 2.69 accidents per million turning vehicles. The total turning volume for this median opening is 500 veh/day, since all turning move- ments are permitted. 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) and U-turns on the major road in Direction 2. Directional Midblock: The accident rate for a directional midblock median opening is computed as follows: Accident Rate acc/yr( ) = × × = 1 40 10 250 365 0 13 6 . . / acc veh veh day days yr acc yr Accident Rate acc/yr( ) = × × = 2 69 10 500 365 0 49 6 . . / acc veh veh day days yr acc yr C-25 From Table C-2, the median opening accident rate for a directional midblock median opening is 0.23 accidents per million turning vehicles. The total turning volume for this median opening is 250 veh/day, since the midblock median opening accommodates major-road U-turns in Direction 1 and left turns from the minor road (Direction 4). Step 4b: Calculate the accident rate for exposure due to additional travel distance In this example, the directional midblock median open- ing is assumed to be located 0.2 mi from the three-leg inter- section. Thus, the accident rate for the exposure related to additional travel distance is computed as follows: From NCHRP Report 282, the average non-intersection accident rate for a four-lane divided highway is 2.90 acci- dents per million-vehicle-miles of travel. The total num- ber of vehicles traveling the additional distance is equal to the total turning volume through the directional median opening, which is 250 veh/day. Step 4c: Calculate the total accident rate for each median opening design alternative Alternative 1: Conventional Median Opening at a Three- Leg Intersection Accident rate (conventional three-leg) = 0.49 acc/yr Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident rate (due to exposure) = ___ (acc/yr) Total accident rate = 0.49 acc/yr Alternative 2: Combination of Directional Median Open- ing at a Three-Leg Intersection and a Direc- tional Midblock Median Opening Accident rate (directional three-leg) = 0.13 acc/yr Accident Rate acc/yr mi( ) = × ×( ) × × = 2 90 10 2 0 2 250 365 0 11 6 . . . / acc veh mi veh day days yr acc yr - Accident Rate acc/yr( ) = × × = 0 23 10 250 365 0 02 6 . . / acc veh veh day days yr acc yr

Accident rate (directional midblock) = 0.02 acc/yr Accident rate (individual median opening) = ___ (acc/yr) Accident rate (due to exposure) = 0.11 acc/yr Total accident rate = 0.26 acc/yr Step 5: Compare accident rates for each median open- ing design alternative Alternative 1 = 0.49 acc/yr Alternative 2 = 0.26 acc/yr In this example, the combination of directional median openings (Alternative 2) represents a 47% reduction in accident rate over the conventional median opening at a three-leg intersection (Alternative 1). Example 2: Comparison of the safety performance of conventional and directional median open- ings at four-leg intersections Step 1: Select median opening design alternatives Alternative 1: Conventional median opening at a four-leg intersection Alternative 2: Directional median opening at a four-leg intersection in combination with two direc- tional midblock median openings Figure C-22 illustrates Alternatives 1 and 2. In Alternative 2, the directional median opening at the four-leg intersec- tion accommodates vehicles making left-turn maneuvers C-26 from the major road onto the minor road and vehicles mak- ing 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. Step 2: Enter all turning volumes Enter all turning volumes for the median opening design alternatives considered: Volume A (Major-road U-turns in Direction 1) = 40 veh/day Volume B (Major-road U-turns in Direction 2) = 40 veh/day Volume C (Major-road left turns in Direction 1) = 180 veh/day Volume D (Major-road left turns in Direction 2) = 180 veh/day Volume E (Minor-road left turns in Direction 3) = 180 veh/day Volume F (Minor-road left turns in Direction 4) = 180 veh/day Volume G (Minor-road throughs in Direction 3) = 100 veh/day Volume H (Minor-road throughs in Direction 4) = 100 veh/day Total turning volume = 1,000 veh/day Step 3: Enter the average non-intersection accident rate for the major road Non-intersection accident rate for the major road = 2.90 acc/MVM (from NCHRP Report 282) a) Conventional Median Opening at Four-Leg Intersection (Type 5b) b) Directional Median Opening Four-Leg Intersection With Two Directional Midblock Median Openings (Type 2b + Type 6a + Type 2b) Figure C-22. Conventional and directional median openings at four-leg intersection.

Step 4: Calculate the accident rate for each median opening design alternative For Alternative 1, only the accident rate for a conven- tional median opening at a four-leg intersection needs to be computed. For Alternative 2, the accident rates for a directional median opening at a four-leg intersection, two directional midblock median openings, and the exposure due to addi- tional travel distance need to be computed. Step 4a: Calculate the accident rate for each individ- ual median opening Conventional four-leg: The accident rate for a conventional median opening at a four-leg intersection is computed as follows: From Table C-2, the median opening accident rate for a conventional four-leg median opening is 3.01 accidents per million turning vehicles. The total turning volume for this median opening is 1,000 veh/day, since all turning movements as well as through movement on the minor road are permitted. Directional four-leg: The accident rate for a directional median opening at a four-leg intersection is computed as follows: From Table C-2, the median opening accident rate for a directional four-leg median opening is 2.57 accidents per million turning vehicles. The total turning volume for this median opening is 440 veh/day, since the only turning movements permitted include left turns from the major road onto the minor road and U-turns on the major road. Directional Midblocks: The accident rate for each of the directional midblock median openings is computed as follows: Accident Rate acc/yr( ) = × × = 2 57 10 440 365 0 41 6 . . / acc veh veh day days yr acc yr Accident Rate acc/yr( ) = × × = 3 01 10 1000 365 1 10 6 . . / acc veh veh day days yr acc yr C-27 From Table C-2, the median opening accident rate for a directional midblock median opening is 0.23 accidents per million turning vehicles. The total turning volume for each median opening is 280 veh/day, since each median open- ing accommodates left-turn maneuvers from one of the minor-road approaches. Step 4b: Calculate the accident rate for exposure due to additional travel distance In this example, each directional midblock median open- ing is assumed to be located 0.2 mi from the four-leg inter- section. Thus, the accident rate for the exposure related to additional travel distance is computed as follows: From NCHRP Report 282, the average non-intersection accident rate for a four-lane divided highway is 2.90 acci- dents per million-vehicle-miles of travel. The total num- ber of vehicles traveling the additional distance is equal to the total turning volumes through both directional median openings, which is 560 veh/day. Step 4c: Calculate the total accident rate for each median opening design alternative Alternative 1: Conventional Median Opening at a Four- Leg Intersection Accident rate (conventional four-leg) = 1.10 acc/yr Accident rate (individual median opening) = ___ (acc/yr) Accident rate (individual median opening) = ___ (acc/yr) Accident rate (due to exposure) = ___ (acc/yr) Total accident rate = 1.10 acc/yr Alternative 2: Combination of Directional Median Open- ing at a Four-Leg Intersection and Two Directional Midblock Median Openings Accident Rate acc/yr mi( ) = × ×( ) × × = 2 90 10 2 0 2 560 365 0 24 6 . . . / acc veh mi veh day days yr acc yr - Accident Rate acc/yr( ) = × × = 0 23 10 280 365 0 02 6 . . / acc veh veh day days yr acc yr

Accident rate (directional four-leg) = 0.41 acc/yr Accident rate (directional midblock) = 0.02 acc/yr Accident rate (directional midblock) = 0.02 acc/yr Accident rate (due to exposure) = 0.24 acc/yr Total accident rate = 0.69 acc/yr C-28 Step 5: Compare accident rates for each median open- ing 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 acci- dent rate over the conventional median opening at a four- leg intersection (Alternative 1).

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Safety of U-Turns at Unsignalized Median Openings Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 524: Safety of U-Turns at Unsignalized Median Openings includes recommended guidelines for locating and designing unsignalized median openings, and a methodology for comparing the relative safety performance of different designs.

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