Complex sentence: In 1964, the GR-1 superhighway was opened to traffic and also additional roads were to be paved, the secondary ones were paved in the 1960s and the tertiary ones were paved in the late-20th century.
Split into simpler sentences: In 1964, the GR-1 superhighway was opened to traffic and also additional roads were to be paved. The secondary ones were paved in the 1960s and the tertiary ones were paved in the late-20th century.