The B138 is a residential B-road in north London. The road was originally unclassified but gained its number around 1930, making it the newest classified road in the area.
The road starts at a mini-roundabout on the B150 Endymion Road on the northern edge of Finsbury Park (the actual park, that is). It heads north along Alroy Road and soon goes over the GOBLIN just east of the East Coast Main Line, which is more visible here and which we are following. We continue north as one of the legs of the Harringey Ladder; it is obvious from a map why this area is so-called. The other leg is the A105 to the east and there are a large number of side streets, alternately one-way eastbound and westbound, forming the rungs between the two.
Presently we reach the top of the Ladder and cross the A504 at traffic lights. There's another set of lights just afterwards to serve the unclassified Clarendon Road which serves an industrial estate. We continue north along Hornsey Park Road, now with none of the side streets managing to reach the A105. The road soon bears round to the right and ends by The Mall shopping centre by turning left and morphing into the B151. Originally the B151 was a through road to the A105 but the shopping centre was built on top of the link to the right.