Entry width
Entry width is the largest determinant of a roundabout’s capacity. The capacity of an approach is not dependent merely on the number of entering lanes, but on the total width of the entry. In other words, the entry capacity increases steadily with incremental increases to the entry width. Therefore, the basic sizes of entries and circulatory roadways are generally described in terms of width, not number of lanes. Entries that are of sufficient width to accommodate multiple traffic streams (at least 6.0 m [20 ft]) are striped to designate separate lanes. However, the circulatory roadway is usually not striped, even when more than one lane of traffic is expected to circulate (for more details related to roadway markings, see Chapter 7).
As shown in Exhibit 6-1, entry width is measured from the point where the yield line intersects the left edge of the traveled-way to the right edge of the traveledway, along a line perpendicular to the right curb line. The width of each entry is dictated by the needs of the entering traffic stream. It is based on design traffic volumes and can be determined in terms of the number of entry lanes by using Chapter 4 of this guide. The circulatory roadway must be at least as wide as the widest entry and must maintain a constant width throughout.
To maximize the roundabout’s safety, entry widths should be kept to a minimum. The capacity requirements and performance objectives will dictate that each entry be a certain width, with a number of entry lanes. In addition, the turning requirements of the design vehicle may require that the entry be wider still. However, larger entry and circulatory widths increase crash frequency. Therefore, determining the entry width and circulatory roadway width involves a trade-off between capacity and safety. The design should provide the minimum width necessary for capacity and accommodation of the design vehicle in order to maintain the highest level of safety. Typical entry widths for single-lane entrances range from 4.3 to 4.9 m (14 to 16 ft); however, values higher or lower than this range may be required for site-specific design vehicle and speed requirements for critical vehicle paths.
When the capacity requirements can only be met by increasing the entry width, this can be done in two ways:
- By adding a full lane upstream of the roundabout and maintaining parallel lanes through the entry geometry; or
- By widening the approach gradually (flaring) through the entry geometry.
Exhibit 6-20 and Exhibit 6-21 illustrate these two widening options.
As discussed in Chapter 4, flaring is an effective means of increasing capacity without requiring as much right-of-way as a full lane addition. While increasing the length of flare increases capacity, it does not increase crash frequency. Consequently, the crash frequency for two approaches with the same entry width will be essentially the same, whether they have parallel entry lanes or flared entry designs. Entry widths should therefore be minimized and flare lengths maximized to achieve the desired capacity with minimal effect on crashes. Generally, flare lengths should be a minimum of 25 m (80 ft) in urban areas and 40 m (130 ft) in rural areas. However, if right-of-way is constrained, shorter lengths can be used with noticeable effects on capacity (see Chapter 4).
In some cases, a roundabout designed to accommodate design year traffic volumes, typically projected 20 years from the present, can result in substantially wider entries and circulatory roadway than needed in the earlier years of operation. Because safety will be significantly reduced by the increase in entry width, the designer may wish to consider a two-phase design solution. In this case, the firstphase design would provide the entry width requirements for near-term traffic volumes with the ability to easily expand the entries and circulatory roadway to accommodate future traffic volumes. The interim solution should be accomplished by first laying out the ultimate plan, then designing the first phase within the ultimate curb lines. The interim roundabout is often constructed with the ultimate inscribed circle diameter, but with a larger central island and splitter islands. At the time additional capacity is needed, the splitter and central islands can be reduced in size to provide additional widths at the entries, exits, and circulatory roadway.