Traffic congestion on motorways on weekends and holidays is not unusual in Japan. To mitigate congestion, installation of an auxiliary lane around the bottleneck in the sag section is used as an alternative and effective countermeasure against congestion to correct overuse of the median lane. However, there are no guidelines on effective installation of an auxiliary lane in the sag section to mitigate motorway traffic congestion. This paper uses a microscopic traffic simulation model to study how best to add an auxiliary lane in the sag section of a dual two-lane motorway to mitigate traffic congestion.