"Depending on agency practice and specific location requirements, medians may be
depressed, raised, or flush with the surface of the traveled way. Medians should have
a dimension that is in balance with the other elements of the total highway cross
section.". This section describes the different kind of median and their relationship to different kind of road configurations.
An important consideration in the design of any multilane highway is whether to provide a median and, if one is provided, what the dimensions should be. The primary functions of highway medians are to:
Separate opposing traffic flows
Provide a recovery area for out-of-control vehicles
Allow space for speed changes and left-turning and U-turning vehicles
Minimize headlight glare
Provide width for future lanes (particularly in suburban areas)
Provide a space for landscape planting that is in keeping with safety needs and improves the aesthetics of the facility
Provide a space for barriers. Depending on agency practice and specific location requirements, medians may be depressed, raised, or flush with the surface of the traveled way. Medians should have a dimension that is in balance with the other elements of the total highway cross section. The general range of median widths is from 1.2 m (4 ft), usually in urban areas, to 24 m (80 ft) or more, in rural areas. An offset of at least a 500 mm (1.5 ft) should be provided between any vertical element located within the median, such as a curb or barrier, and the edge of the adjacent traveled lane.
The design and width of medians again require tradeoffs for designers. In locations where the total available right-of-way is restricted, a wide median may not be desirable if it requires narrowing the areas adjacent to the outside edge of the traveled way. A reasonable border width is required to serve as a buffer between private development along the road and the edge of the traveled way, and space may be needed for sidewalks, highway signs, utilities, parking, drainage channels and structures, proper slopes and clear zones, and any retained native plant material. On the other hand, wider medians provide more space for plant material, offer a refuge for pedestrians at intersections, and help soften the look of the roadway. Including and designing medians requires public input to find the design that meets the needs of the community.
The use of two-way left-turn lanes on urban streets in densely developed suburban commercial areas has increased as an alternative to raised medians with left-turn or U-turn bays. Although not as aesthetically pleasing as raised, planted medians, continuous left-turn lanes can improve capacity. Two-way left-turn lanes generally are not recommended in residential areas because they do not afford a safe refuge for pedestrians. Also, the number of driveways can create unsafe vehicle maneuvers.
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