When must headlights be used in seriously reduced visibility in Northern Ireland?

You must use headlights when visibility is seriously reduced, generally when you cannot see for more than about 100 metres.

This NI Highway Code rule applies to fog and other severe visibility reduction, not just darkness.

For “When must headlights be used in seriously reduced visibility in Northern Ireland?”, the wider Northern Ireland context is this: Northern Ireland winter driving is especially sensitive to changing road-surface temperature, rain, ice, flooding and exposed routes. Official advice is to drive for the conditions actually in front of you, not for the posted speed limit or the fact that a road was treated earlier. TrafficWatchNI and current weather warnings are the right live sources when conditions are deteriorating.

For this exact question — “When must headlights be used in seriously reduced visibility in Northern Ireland?” — the practical point is: Switch lights on early enough to be seen and reduce speed to a level that lets you stop within the distance you can see to be clear. The answer should be applied to the actual signs, conditions, licence wording or parking rules in force at the time, because temporary restrictions and individual DVA decisions can change the ordinary position.

This Northern Ireland-specific page on “When must headlights be used in seriously reduced visibility in Northern Ireland?” was checked against the attached official sources before publication, with nidirect/DVA and the Northern Ireland Highway Code used ahead of generic UK motoring articles. If the rule, road layout or personal licence condition changes, those primary sources are the ones to re-check.