Alignment of approaches and entries

In general, the roundabout is optimally located when the centerlines of all approach legs pass through the center of the inscribed circle. This location usually allows the geometry to be adequately designed so that vehicles will maintain slow speeds through both the entries and the exits. The radial alignment also makes the central island more conspicuous to approaching drivers.

If it is not possible to align the legs through the center point, a slight offset to the left (i.e., the centerline passes to the left of the roundabout’s center point) is acceptable. This alignment will still allow sufficient curvature to be achieved at the entry, which is of supreme importance. In some cases (particularly when the inscribed circle is relatively small), it may be beneficial to introduce a slight offset of the approaches to the left in order to enhance the entry curvature. However, care must be taken to ensure that such an approach offset does not produce an excessively tangential exit. Especially in urban environments, it is important that the exit geometry produce a sufficiently curved exit path in order to keep vehicle speeds low and reduce the risk for pedestrians.

It is almost never acceptable for an approach alignment to be offset to the right of the roundabout’s center point. This alignment brings the approach in at a more tangential angle and reduces the opportunity to provide sufficient entry curvature. Vehicles will be able to enter the roundabout too fast, resulting in more loss-of-control crashes and higher crash rates between entering and circulating vehicles. Exhibit 6-18 illustrates the preferred radial alignment of entries.

In addition, it is desirable to equally space the angles between entries. This provides optimal separation between successive entries and exits. This results in optimal angles of 90 degrees for four-leg roundabouts, 72 degrees for five-leg roundabouts, and so on. This is consistent with findings of the British accident prediction models described in Chapter 5.