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20 Median acceleration lanes create conflicts. enced a high percentage of fixed-object and sideswipe crashes. Median acceleration lanes are unexpected and unfamil- Specifically, the following safety concerns were found at loons: iar to drivers. The benefits of median acceleration lanes do not warrant Fixed-object crashes with delineator posts, sign posts the construction costs. (in the median and along the mainline), and guardrail; Sideswipe crashes involving vehicles merging into main- The agencies also stated that median acceleration lanes are line traffic from the loon; most effective at high-speed T-intersections on rural roads. Sideswipe crashes involving mainline traffic attempting Median acceleration lanes can improve the operation of to use the right-turn lane and crashing with U-turning directional median openings by helping U-turning vehicles vehicles that turned from the crossover into the loon and to accelerate and merge with traffic on the through roadway. then proceeded directly into the right-turn lanes; and There are no data on whether median acceleration lanes at Commercial vehicles backing up and parking within the conventional median openings create additional conflicts for crossover. drivers making U-turn maneuvers. An operational analysis concluded that loons provide com- mercial vehicles with the extra pavement necessary to com- LOONS TO ASSIST LARGER VEHICLES IN COMPLETING U-TURN MANEUVERS plete the U-turn maneuver required by indirect left-turns along narrow medians. Use of advance warning signs to improve A common problem associated with the use of directional driver expectancy is recommended. Finally, the authors crossovers for indirect left turns is the difficulty of larger present guidelines for the design and placement of loons. vehicles to negotiate U-turns along cross-sections with nar- row medians. This situation often affects the operation and safety of commercial vehicles that typically require more SIGHT DISTANCE AT MEDIAN OPENINGS space in order to perform a U-turn maneuver. One possible solution to this problem is the construction of a loon. Loons Intersection sight distance (ISD) is an important design are defined as expanded paved aprons opposite a median and operational consideration at all intersections, but may be crossover. Their purpose is to provide additional space to even more important at divided highway intersections, includ- facilitate the larger turning path of commercial vehicles along ing unsignalized median openings, where the median may narrow medians. Figure 7 presents a typical loon design. increase the ISD requirements or may contain sight obstruc- The genesis of the term "loon" is not clear, but it appears tions that reduce the ISD. U-turn maneuvers should not be to be coming into common use. Loons appear to have been encouraged at locations with limited sight distance. used at directional median openings, but the concept may be Both NCHRP Report 383: Intersection Sight Distance (57) applicable to conventional median openings as well. and NCHRP Report 375 (7), identify situations where ISD A study by Sisiopiku and Aylsworth-Bonzelet (55, 56) requirements for divided highway intersections may differ evaluated the operation, placement, and safety of existing from undivided highway intersections. NCHRP Synthesis of loons at directional median openings in western Michigan. Practice 281: Operational Impacts of Narrow Medians on The Michigan DOT has placed several loons along a 47-km Larger Vehicles (54) identifies sight distance as an important (29-mi) corridor of divided roadway to facilitate the larger issue in determining locations where U-turns by larger vehi- turning radii of commercial vehicles performing indirect left cles should be permitted or encouraged. The Florida Median turns. Field data (including geometrics, posted speed limits, Handbook (8) also addresses sight distance issues at median sign types and location, and traffic control) and 5 years' of openings. accident data were collected for the analysis. Results of the NCHRP Report 383 (57) presents revised ISD models that study indicate that directional crossovers with loons experi- have been adopted by AASHTO and incorporated into the Figure 7. Typical loon design at a directional median opening (55, 56).