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Tower Bridge, London UK Tower Bridge, London

On the London Bridge they are positioned such that they look like barriers, but here as pictured on the Tower bridge they don't seem to offer any such protection.

What are these things and what purpose do they serve?

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They prevent cars ands trucks from driving onto the pavement and carrying out attacks against pedestrians, similar to what happened during the 2017 Westminster Attack and the 2017 London Bridge Attack

Images of the same barriers in use for similar reasons:

Anti-vehicle barriers in place on London Bridge

Source: Wikimedia

Anti-vehicle barriers in place at Windsor Castle

Source: Reuters

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    That's great, they look very strong and initially that's what I thought but their location on the tower bridge was somewhat confusing so I thought they might be something else. They look historical too which added to my confusion – Hanky Panky Sep 13 '17 at 4:51
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    @HankyPanky they went up everywhere across London after the attacks earlier this year - but some may have been in place prior to that as Boris Johnson removed a load of anti-vehicle "ugly" barriers during his reign as Mayor of London. – Moo Sep 13 '17 at 4:52
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    Any idea why are they connected to each other by metal pipes in some places, like in your first picture, and they aren't in some other places like I saw on London Bridge. That doesn't help pedestrians does it – Hanky Panky Sep 13 '17 at 6:05
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    @HankyPanky multiple units are tied together to increase their stopping power - one of those alone might not stop a large truck at 30mph, but it would stop a similar truck at a lower speed. There simply isnt enough space there to put multiple barriers down side by side without impeding the flow of pedestrians, and since that bridge is a major tourist attraction it becomes a trade off - the average speed over that bridge at any time is fairly low, whereas in other locations an attacking vehicle may be able to gain some serious speed and get through a single barrier. – Moo Sep 13 '17 at 6:10
  • @HankyPanky in my first example picture, there is enough gap left either side of the three to allow rush hour pedestrian traffic, but that also means that if a truck or car aims for the gap, they are only going to hit one of the three barriers - so tie them together. You cant decrease the gap without it becoming a significant bottleneck in peak pedestrian rush hour, so you ensure that hitting one barrier means hitting all three. – Moo Sep 13 '17 at 6:12

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