The B1520 follows the pre-bypass route of the A16 through Louth.
The road starts at a roundabout at the southern end of the bypass, where it also meets the A157. It heads north into town along London Road, remaining largely rural until becoming built-up unexpectedly on the edge of the town centre. The B1200 (pre-bypass A157) is crossed at traffic lights and then we wind through the centre of town. The road past the church is narrow enough that flow must be controlled by traffic lights, which also controls access to/from a side street after the S1 section ends.
We continue along Bridge Street and cross the River Lud, after which a spur of the B1200 comes in from the left. We follow Grimsby Road through the northern suburbs. We TOTSO left at a T-junction and end at a roundabout shortly afterwards, where we rejoin the A16.