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.