Complex sentence: It used to have entirely a 2-lane highway and ended near Katerini, in the mid- 1970s, it extended to Thessaloniki and to the Republic of Macedonia border, the highway had 4-lanes.
Split into simpler sentences: It used to be entirely a 2-lane highway and ended near Katerini in the mid- 1970s. When it extended to Thessaloniki and to the Republic of Macedonia border, the highway had 4-lanes.