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S4 singe carriageway on Craigavon Bridge, Derry.

S4 refers to a single carriageway road with four lanes. Usually it is marked out as two lanes for each direction. Sometimes the centre of the road is marked with double white solid lines to prevent overtaking across the centre line.

In Ireland, the 1998 Road Needs Study described S4 as "extremely dangerous" when used outside of slow-moving urban areas.


Explanation of S4 on CBRD (

SABRE - The Society for All British and Irish Road Enthusiasts
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