Roundabout Properties: Arm: Splitter Island

The splitter island is distinct from the splitter. The splitter is formed by the triangle of entry, exit and roundabout definition lines and is normally marked on the road. A splitter island typically has a curbed edge and may or may not have pedestrian access and/or a refuge. In some cases, splitter islands are mandatory, in others they are discretionary.

Expand an arm and click Splitter Island in the left hand pane. The Splitter Island page is displayed.

The splitter island page is divided into three identical areas that relate to each of the three sides of the splitter island and a fourth that relates to the fillet radii.

To add or remove the splitter island, use the Splitter Island checkbox.

The Island Length typically is displayed in red, indicating that it has been calculated from the other geometry. If you set a fixed length that is shorter than the other geometry will allow, the island is truncated. If you set a fixed length that is longer than the other geometry will allow, the island is drawn as long as possible within the geometry.

Each of the three curb lines may either be straight or curved. Select Curved to specify a curved line. This enables the Curb Radius field.

The Curb Radius is set by default to Tracking. This means that it is offset from the splitter definition line and that the offset changes linearly from the inner offset to the outer offset. If you wish to specify a fixed radius then enter a value.

Note: If the end offsets are equal then the curb radius is a circular arc concentric with the splitter definition line.

The Inner Offset is the distance by which the splitter island is offset from the splitter definition line at the end closest to the center of the roundabout.

The Outer Offset is the distance by which the splitter island is offset from the splitter definition line at the end furthest from the center of the roundabout.

The Entry Offset and Exit Offset on the Roundabout Curb Line section are the distances by which the splitter island are offset from the inscribed circle at the entry and exit ends respectively.

The Entry Fillet Radius is the radius of the fillet at the intersection of the entry curb line and the roundabout curb line.

The Exit Fillet Radius is the radius of the fillet at the intersection of the exit curb line and the roundabout curb line.

The Approach Fillet Radius is the radius of the fillet at the intersection of the entry and exit curb lines.

Note: If the approach road has a central gap and the sum of the outer offsets at entry and exit is less than the central gap width then the island approach fillet is omitted and the splitter island curb lines are continued to form a central reservation.

In all cases the Max and Min values represent the limiting values from the adopted design standard.