The older highways, constructed before travel demand on those routes was high or before the legislation of control of access are often lined with residences, businesses, and farms.
In practice, road twinning projects are usually impractical in more populated regions and are more typically done through non-agricultural corridors (except in cases where the twinned highway is not grade-separated and allows driveway access) with fewer settlements and have sections of new bypasses built around the settlements that do exist.
The bypass (or sometimes an entirely-new highway) may be initially designed as a freeway, but often, the need for such a route is not immediately present.
A wide right-of-way is purchased in most cases so that enough property is available to simply twin the highway.
Twinning is often a public preference to reduce the dangers of overcrowded two-lane single carriageways, like sections of the Trans-Canada Highway.