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32 stated that they have no policy or use engineering judgment. Highway agencies were asked whether they had experi- The remaining agencies that responded have specific policies enced safety or traffic operational problems at unsignalized that differ from AASHTO Green Book; these policies are median openings. Nearly 60 percent of the agencies that summarized below: responded indicated that they had. The five most cited fac- tors related to safety or operational problems at unsignalized · Minimum median widths wider than those presented in median openings in decreasing order are as follows: the Green Book are used. · U-turns are permitted on roads with a minimum width · Operational considerations (e.g., heavy U-turns or through of six lanes. volumes, and trucks), · Minimum median widths in the range of 3 to 6 m (10 to · Median too narrow, 20 ft), based on the type of roadway, are used. · Driveway nearby, · U-turns are permitted on arterials with a minimum width · Poor roadway geometry, and of 12.8 m (42 ft), which includes a 1.2-m (4-ft) separa- · Roadway too narrow. tor, 10 m (33 ft) of travel lane width, and 1.5 m (5 ft) of bike lane width. A more complete list of factors identified by highway agen- · U-turns are permitted with a median width of 4.3 m cies as related to the safety or operational problems they (14 ft) and roadway width for one direction of travel of encountered at unsignalized median openings is presented in 7.9 m (26 ft). Appendix B. · U-turns are permitted with a median width between 6 and 7 m (20 and 24 ft) and directional roadway width between 9 and 11 m (30 and 36 ft) for a four-lane divided MITIGATION MEASURES FOR SAFETY highway. PROBLEMS When asked whether their criteria for design of median When asked if they have constructed improvement proj- openings included provisions for U-turns by large vehicles ects intended to mitigate safety and operational problems at (e.g. school buses, other buses, or large trucks), approximately unsignalized median openings, 37 percent of responding high- one-half of the responding state agencies and one local agency way agencies indicated they have constructed improvement indicated that such provisions are made. projects. Some of the mitigation measures for safety and operational problems cited are as follows: TRAFFIC OPERATIONAL AND SAFETY · Removal of closely spaced median openings by replac- PROBLEMS AT MEDIAN OPENINGS ing raised medians with TWLTLs; · Replacement of conventional crossovers with directional Several factors affect the safety and operational perfor- crossovers; mance of a median opening, e.g., · Installation of left-turn storage lanes; · Installation of left-turn lanes with positive offset; · Degree of urbanization, · Signalized intersections; · Operating speeds, · Closure of median openings to allow left turns to align · Access density, · Roadway geometrics, properly; · Traffic volumes, · Reconfiguring of median openings by channelizing or · Physical constraints, and adding left-turn lanes to prevent congestion or confu- · Median width. sion in the median opening; · Installation of directional median openings to permit For example, it is well known that locations with fewer con- left turns from the major-road left-turn lane, but prohibit flict points (i.e., where fewer traffic movements cross one left turns and through movements from the minor road; another) are likely to experience fewer accidents than loca- · Provision of a median opening at what formerly was a tions with more conflict points. Thus, it is likely that a median right-in/right-out driveway; opening that serves U-turns only will operate more safely than · Elimination of conventional median openings and one where U-turns use the same roadway as left-turn and replacement with jughandle U-turns; crossing maneuvers. Similarly, a median opening that serves · Installation of "No U-turn" signs; and only one intersection or driveway would function like a three- · Installation of raised/extended median to prevent U-turns. leg intersection and would likely operate more safely than a median opening that serves intersection legs or driveways on Six of the agencies that constructed improvement projects both sides of the arterial roadway, which would function like indicated that they have conducted a formal before-and-after a four-leg intersection. evaluation.