"

Thanks.  I see that a particular type of interchange, or any interchange at all, is not an essential feature of twinning.  I suggest the present sentence should be replaced by something like:
This section of the route involved constructing a new carriageway parallel to the existing Highway 16, a process known as twinning.  Uninterrupted cross-flow of traffic was achieved by grade-separated interchanges.
 (t) "