Vertical considerations

Elements of vertical alignment design for roundabouts include profiles, superelevation, approach grades, and drainage.

Profiles

The vertical design of a roundabout begins with the development of approach roadway and central island profiles. The development of each profile is an iterative process that involves tying the elevations of the approach roadway profiles into a smooth profile around the central island.

Generally, each approach profile should be designed to the point where the approach baseline intersects with the central island. A profile for the central island is then developed which passes through these four points (in the case of a fourlegged roundabout). The approach roadway profiles are then readjusted as necessary to meet the central island profile. The shape of the central island profile is generally in the form of a sine curve. Examples of how the profile is developed can be found in Exhibits 6-34, 6-35, and 6-36, which consist of a sample plan, profiles on each approach, and a profile along the central island, respectively. Note that the four points where the approach roadway baseline intersects the central island baseline are identified on the central island profile.

Superelevation

As a general practice, a cross slope of 2 percent away from the central island should be used for the circulatory roadway. This technique of sloping outward is recommended for four main reasons:

• It promotes safety by raising the elevation of the central island and improving its visibility;
• It promotes lower circulating speeds;
• It minimizes breaks in the cross slopes of the entrance and exit lanes; and
• It helps drain surface water to the outside of the roundabout (2, 6).

The outward cross slope design means vehicles making through and left-turn movements must negotiate the roundabout at negative superelevation. Excessive negative superelevation can result in an increase in single-vehicle crashes and loss-of-load incidents for trucks, particularly if speeds are high. However, in the intersection environment, drivers will generally expect to travel at slower speeds and will accept the higher side force caused by reasonable adverse superelevation (10).

Exhibit 6-37 provides a typical section across the circulatory roadway of a roundabout without a truck apron. Exhibit 6-38 provides a typical section for a roundabout with a truck apron. Where truck aprons are used, the slope of the apron should be 3 to 4 percent; greater slopes may increase the likelihood of loss-of-load incidents.

Locating roundabouts on grades

It is generally not desirable to locate roundabouts in locations where grades through the intersection are greater than four percent. The installation of roundabouts on roadways with grades lower than three percent is generally not problematic (6). At locations where a constant grade must be maintained through the intersection, the circulatory roadway may be constructed on a constant-slope plane. This means, for instance, that the cross slope may vary from +3 percent on the high side of the roundabout (sloped toward the central island) to -3 percent on the low side (sloped outward). Note that central island cross slopes will pass through level at a minimum of two locations for roundabouts constructed on a constant grade.

Care must be taken when designing roundabouts on steep grades. On approach roadways with grades steeper than -4 percent, it is more difficult for entering drivers to slow or stop on the approach. At roundabouts on crest vertical curves with steep approaches, a driver’s sight lines will be compromised, and the roundabout may violate driver expectancy. However, under the same conditions, other types of at-grade intersections often will not provide better solutions. Therefore, the roundabout should not necessarily be eliminated from consideration at such a location. Rather, the intersection should be relocated or the vertical profile modified, if possible.

Drainage

With the circulatory roadway sloping away from the central island, inlets will generally be placed on the outer curbline of the roundabout. However, inlets may be required along the central island for a roundabout designed on a constant grade through an intersection. As with any intersection, care should be taken to ensure that low points and inlets are not placed in crosswalks. If the central island is large enough, the designer may consider placing inlets in the central island.