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6 openings designed to accommodate vehicles making U-turns The Green Book makes the following recommendations only are needed on some divided highways in addition to on the spacing of median openings (3): openings provided for cross and left-turning movements. Separate U-turn median openings may be needed at the fol- · Spacing between median openings should be adequate lowing locations: to allow for introduction of left-turn lanes. · Median openings should reflect street or block spacing · Locations beyond intersections to accommodate minor and the access classification of the roadway. turning movements not otherwise provided in the inter- · Full median openings should be consistent with traffic section or interchange area. The major intersection area signal spacing criteria. is kept free for the important turning movements, in some · Spacing of openings should be consistent with access cases obviating expensive ramps or additional structures. management classifications of criteria. · Locations just ahead of an intersection to accommodate U-turn movements that would interfere with through Research reported in NCHRP Report 348 (5) indicates that and other turning movements at the intersection. Where several states have set median opening spacing criteria that a fairly wide median on the approach highway has few range from 100 to 800 m (330 to 2,640 ft). These criteria are openings, U-turns are necessary for motorists to reach mainly applicable in suburban and rural environments. The roadside areas. Advancing separate openings to accom- report also presents minimum desired spacing of unsignal- modate them outside the intersection proper will reduce ized median openings at driveways as a function of speed. interference. This spacing ranges from 113 m (370 ft) at 48 km/h (30 mph) · Locations occurring in conjunction with minor cross- to 278 m (910 ft) at 88 km/h (55 mph). In addition, the report roads where traffic is not permitted to cross the major suggests the following guidelines be considered for the spac- highway but instead is required to turn right, enter the ing and design of median openings on divided roadways: through traffic stream, weave to the left, U-turn, and then return. On high-speed or high-volume highways, the dif- · The spacing of median openings for signalized drive- ficulty of weaving and the long lengths involved usu- ways should reflect traffic signal coordination require- ally make this design pattern undesirable, unless the ments and the storage space needed for left turns. volumes intercepted are light and the median is of ade- · The spacing of median openings for unsignalized drive- quate width. This condition may occur where a crossroad ways should be based on a roadway's function or access with high-volume traffic, a shopping area, or other traf- level and the environment in which the roadway is located fic generator that needs a median opening nearby and (e.g., rural) and should be conducive to signalization. additional median openings would not be practical. · Median openings for left-turn entrances should be spaced · Locations occurring where regularly spaced openings to allow sufficient storage for left-turning vehicles. facilitate maintenance operations, policing, repair service · Median openings at driveways could be subject to clo- of stalled vehicles, or other highway-related activities. sure where volumes warrant signals, but signal spacing Openings for this purpose may be needed on controlled- would be inappropriate. access highways and on divided highways through unde- · Median openings should be set back far enough from veloped areas. nearby signalized intersections to avoid possible interfer- · Locations occurring on highways without control of ence with intersection queues, and storage for left turns access where median openings at optimum spacing are must be adequate. provided to serve existing frontage developments and at the same time minimize pressure for future median TRB Circular 456 (6) indicates that median openings gen- openings. A preferred spacing at 400 to 800 m (0.25 to erally should relate to the street or block spacing. Thus, where 0.50 mi) is suitable in most instances. Fixed spacing is cross-streets are placed at regular intervals, these intervals not necessary, nor is it fitting in all cases, because of will influence median opening spacing. The Circular recom- variations in terrain and local service needs. mends that access points on both sides of the road should be aligned on undivided highways. Where this is not possible, SPACING OF MEDIAN OPENINGS sufficient left-turn storage should be provided by establish- ing a minimum offset distance. Driveways should be offset Increasing the spacing between median openings improves from median openings by the following: arterial flow and safety by reducing the number of conflicts and conflict points per mile, providing greater distance to · At least 60 m (200 ft) when two low-volume traffic gen- anticipate and recover from turning maneuvers, and provid- erators are involved, ing opportunities for use of turn lanes (4). It is increasingly · The greater of 60 m (200 ft) or the established median recognized that spacing standards for unsignalized access opening spacing interval when one major traffic gener- points should complement those for signalized access points. ator is involved, and