Kev wrote:

I have only just noticed that it is 20mph across Tower Bridge so are
they some sort of speed camera. What are the cameras on a gantry on
the northern approach for, they look congestion charging cameras,
certainly not speed cameras.

I'd assumed that the gantry cameras were speed cameras or for the control
room to monitor traffic queues. They went up when I worked down there, which
pre-dates congestion charging by some time - apart from which they are
outside the CC zone which (IIRC) begins at the bottom of Minories. As John
mentions, the cameras inside are a new development - perhaps they are
replacing the gantry cameras with a less visible, more aesthetically
pleasing system?