When can a driver legally pass traffic on the left in Northern Ireland?

A driver may pass traffic on the left on a Northern Ireland motorway when congested lanes are moving at different speeds and you simply keep up with the traffic in your own lane.

In Northern Ireland, the legal rule and the practical safety advice need to be read together. That exception does not justify weaving into the left lane to overtake a slower vehicle and then returning right. Normal motorway overtaking remains on the right.

For this exact issue — “When can a driver legally pass traffic on the left in Northern Ireland?” — the Northern Ireland motorway rule has to be applied with high-speed safety in mind. Check the live signs, lane status and whether you are dealing with normal driving, congestion or a genuine emergency. Hard shoulders, emergency areas and motorway lanes are controlled spaces, so actions that would be ordinary on a local road can be prohibited or extremely dangerous here.

Maintain lane discipline and avoid rapid lane changes in queues. The safest way to apply this is to focus on the exact vehicle, road layout and current signage rather than relying on what normally happens at that location.

Before publishing this answer, the key factual point in “When can a driver legally pass traffic on the left in Northern Ireland?” was checked against current Northern Ireland official guidance. That matters because road rules, licence entitlements and enforcement details can differ from Great Britain and can change over time; the sources attached below are the ones to re-check if the rule is updated.