Entry width
Entry angle
Entry radius
Entry deflection
Inscribed circle diameter (ICD)
Circulatory carriageway
Main central island.
Entry width
TD16/93 states that it is good practice to add at least an extra lane to the lanes
on the entry approach. (This has been done at all entry arms in the above
example. In all cases one lane has been widened to two at the entry point.)
Entry angle
It is recommended that the entry angle should be between 10 and 60°. (In the
above example, an entry angle of 30º was used at all entry points.)
Entry radius
The absolute minimum radius should be 6 m and, in order to cater for heavy
goods vehicles, it should preferably not be less than 10 m.
Entry deflection
This is one of the main determinants of safety on a roundabout, indicating the
deflection to the left imposed on all entering vehicles. For each entry arm, the
tightest radius of the entry path curvature, measured over a distance of at least
20–25 m, should not exceed 100 m.
Inscribed circle diameter (ICD)
In order to accommodate the turning movement of a standard 15.5 m long arti-
culated vehicle, the inscribed circle diameter must be at least 28 m. The ICD in
the above example is exactly 28 m.
Circulatory carriageway
The width of the circulatory should not exceed 15 m and should lie between
1.0 and 1.2 times the maximum entry width. The chosen design sets the
circulatory carriageway at 9 m, approximately 1.2 times the maximum entry
width value (7.6 m).
The Design of Highway Intersections 131