In message , Basil Jet
writes

At the point where the road used to go over the canal basin, the road still
has a lengthy bump, but the pavements are banked up a foot or two higher at
the centre.

At a guess, there are utilities running under the pavement that were too
expensive to move - sewer pipes and the like.

In fact, when the canal basin was part filled-in in the 1980s, a large
electricity substation was built adjacent to the bridge. Islington have
a masterplan for redeveloping the area, which mentions the difficulties
caused by high-voltage cables (protected by a grid of pre-cast concrete)
in the immediate locality of the bridge. These include "branches feeding
a network of cable routes running below City Road".
--
Paul Terry