Why can a road with street lights have a 30 mph limit in Northern Ireland?

In Northern Ireland, a road with a system of street lighting will usually be subject to a 30 mph limit unless signs show another limit.

The presence of street lighting can indicate the restricted-road limit even where repeated 30 mph signs are not present. Drivers should still check entry signs and any signed exceptions.

For this type of DfI roads issue, record the exact road name, direction, nearest junction or property and, where relevant, the sign or marking involved. If there is an immediate danger to traffic, prioritise safety and use the appropriate urgent reporting route rather than waiting for a routine roads response. For this question — "Why can a road with street lights have a 30 mph limit in Northern Ireland?" — that location evidence helps DfI distinguish a general request from a specific road defect or restriction.