Twinning a road involves the construction of a similar or identical parallel road. It is usually done when an existing highway requires a significant increase in capacity. Twinning is frequently advantageous because it allows traffic capacity to be doubled and produces a dual carriageway with separation between traffic directions, while keeping the existing right of way. Additionally, unlike simple widening, the old road stays open while the twin is being built.
Some freeways are constructed by bypassing an established two- or four-lane highway. The older highways, constructed before the demand to travel that route existed, or before the legislation of control of access, are often lined with residences and businesses. Without control of access, a municipal or state government is obliged to provide a driveway access where the owner sees fit.
The bypass may be initially designed as a freeway, but often the need for such a route is not immediately present. Instead, a two lane road with no properties abutting it is constructed. A wide right-of-way is purchased in most cases so that enough property is available to simply twin the highway. These roads are known as a Super Two, and can be easily upgraded to a freeway by constructing interchanges and by twinning the road.
Twinning is often a public preference to reduce the dangers of overcrowded 2-lane single carriageways, like sections of the Trans-Canada Highway.