 The first section of the northern half of Ringway 3 was constructed between South Mimms and Potters Bar and opened in 1975.The first section of Ringway 4 was built between Godstone and Reigate and opened the following year.Before the first of these opened, the planned north and east sections of Ringway 3 and the planned south and west sections of Ringway 4 were combined as the M25 (the northern part was initially designated as the M16 during the planning stages but opened as the M25).