Black hole under the river Thames

From New Scientist‘s Feedback column:

CAMPAIGNING for another bridge or tunnel across the Thames river in London, presents meters showing the traffic flowing into and out of each portal of the existing Blackwall tunnel. Checking the site at 5 pm on a Friday, Feedback was concerned to see huge numbers of vehicles entering the northern portal and none at all leaving the southern. What was their fate? Equally worrying, nothing was shown entering from the south, even though traffic was pouring out of the north portal. Presumably these vehicles originated somewhere even more mysterious than south London.

However, as Richard Knight points out, beneath the meters is written “Accuracy +/- 90%”. All may be explained.