Complex sentence: Highway 420 was originally part of the QEW until 1972, when the new QEW extension to Fort Erie, Ontario opened, bypassing the old section which was redesignated as 420.
Split into simpler sentences: Highway 420 was originally part of the QEW. In 1972, the new QEW extension to Fort Erie, Ontario opened, bypassing the old section which was redesignated as 420.