Complex sentence: A comprehensive express subway network, the RER, was built to complement the Métro and serve the distant suburbs, while a network of freeways was developed in the suburbs, centred on the Périphérique expressway encircling the city, completed in 1973.
Split into simpler sentences: A comprehensive express subway network, the RER, was built to complement the Métro and serve the distant suburbs. A network of roads was developed in the suburbs centred on the Périphérique expressway encircling the city, which was completed in 1973.