We've probably all seen buses with advertisements on them -- and very often social statements, paid for by certain interest groups.

But in the US, these kind of socio/political advertisements all tend to be... on the conservative side. "Teach your kids 'virginity' is not a dirty word!" Or, "People should get married" or whatever

In contrast, this is what you're likely to see in London:

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"The favorite American pastime is not baseball, it's moral crusades."