A human shield is a system used by evil dictators to protect their installations from bombing. I don't know much about that.

However, I do know about crossing the road in London's busy streets in tourist season, and it seems to me that the term "human shield" applies. All you need to do is stand on the left of the large group of tourists who will have formed up into a garishly-coloured group while waiting to cross.

Now all you need to do is wait until they think it's safe to cross, which is much quicker than waiting for it to actually be safe. Now as they amble across, you are downstream of them, and any oncomming traffic will either stop by breaking sharply or by dissipating its momentum into the tourist horde.

Either way, you're safe.