The B1512 followed the pre-bypass route of the A1122 through Downham Market. However, the road was short-lived, existing from the construction of the bypass in the 1980s to its declassification in the 1990s.
The road started on the A10 eastern bypass, where the A1122 disappears into a multiplex along that road before turning into the southern bypass. The B1512 headed west along Bexwell Road into the town centre where it met the B1507 by the church. Officially it crossed the B1507 and ran along the High Street and Bridge Street but owing to a one-way system that route could only be followed eastbound. Westbound traffic had to multiplex south along the B1507 Church Road before doubling back on itself to head west along Priory Road.
After eastbound and westbound routes of the B1512 rejoined the road headed west along Railway Road. On the edge of town the road went over a level crossing by the station, ending shortly afterwards at a roundabout back on the A1122.