About Constraints - Civil 3D 2020

Constraints are one of the benefits of grip-editing over manual editing in the Roundabout Properties dialog.

In general, elements are moved independently of other features wherever possible. However, this often requires associated dimensions to change. For example, adjusting the center island diameter forces the outermost circulatory lane width to change.

If you hold down the <Ctrl> key while moving a grip, other elements are constrained to move at the same time. So, in the example above, changing the center island diameter with the <Ctrl> key down also changes the apron and inscribed circle diameter to change while the lane widths remain the same.

Grip	Effect without <Ctrl>	Effect with <Ctrl>
Inscribed Circle Grip	Outermost circulatory lane width changes. Inner limit, outermost circulatory lane width zero. Outer limit, blend point.	Lane widths remain constant and central island and apron change size. Inner limit, center island radius zero. Outer limit, blend point.
Center Island Grip	Outermost circulatory lane width changes. Inner limit, center island radius zero. Outer limit, outermost lane width zero.	Lane widths remain constant and apron and inscribed circle size change. Inner limit, center island radius zero. Outer limit, inscribed circle at blend point.
Apron Width Grip	Outermost circulatory lane width changes. Inner limit, apron width zero. Outer limit, outermost circulatory lane width zero.	Lane widths remain constant and inscribed circle changes size. Inner limit, apron width zero. Outer limit, inscribed circle at blend point.
Lane Grips	Outermost circulatory lane width changes. Inner limit, lane width zero. Outer limit, outermost circulatory lane width zero.	All lane widths outside remain constant and inscribed circle changes size. Inner limit, lane width zero. Outer limit, inscribed circle at blend point.
Road Width Grip	Road width on one side of centerline only changes.	Road widths on both sides of central divide change.
Center Gap Width Grip	Lane widths change as center gap width changes.	Lane widths remain constant as gap width changes.