The portion of the freeway between the mainlanes and frontage roads or ROW where frontage roads are not provided, should be wide enough to accommodate shoulders, auxiliary lanes, side slopes, drainage, retaining walls, ramps, signs, and other appurtenances necessary for traffic control.

Because of ROW limitations in urban areas, the outer separation may oftentimes be less than desired; however, in rural areas, where oncoming headlights along a two-way frontage road tend to reduce a driver’s comfort and perception on the freeway, the outer separation should be as wide as possible. Clear zones, as defined in , are preferable for errant vehicle recovery and safety.

The outer separation should be as wide as economically feasible to provide a buffer zone between the freeway and its adjacent area. Where practical, the outer separation should extend beyond the construction limits when frontage roads are not provided.

The widths of the outer separation will typically range from 80 to 150-ft in rural areas.

Urban areas will typically provide lesser widths, especially when retaining walls are used.