The B1, the lowest-numbered Class II road in the UK, is a short road in east County Down.
The road starts at a TOTSO on the A2 in the coastal village of Ardglass and heads northwestwards out of town, soon reaching open country. The road is fairly straight and goes through no places of any size on its route.
Eventually the B1 reaches Downpatrick. After passing the Downshire Hospital the road TOTSOs left at a T-junction. The next road to the right is the original 1923 line of the road but it was rerouted early on to avoid Edward Street and on to a less-built-up route. As such the B1 descends Ardglass Road to another T-junction where it TOTSOs right; the road to the left is the B176.
Just round the next corner a set of traffic lights is met. At one time the B1 continued ahead along Irish Street into the town centre but now the road markings clearly show that the B1 bears left onto St Patrick's Avenue. This goes through the suburbs to end at traffic lights on the A25.