Someone once told me it's something to do with stopping horses from seeing the daylight at the exit and bolting
That is also suggested for the original Blackwall bore, but I don't know whether it has any validity beyond being a well established urban legend.
I can get you a horse if you want to test it....
if you want to test it
AFAIK, horse drawn traffic is now barred from using either tunnel, so you need to get me not just a horse but a dispensation from TfL 🙂
@Dibble started
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That is also suggested for the original Blackwall bore, but I don't know whether it has any validity beyond being a well established urban legend.