Here's a picture I took of some Police in one of those situations.
I know full well what it means to be in reach of the long arm of the law. Even so, I almost thought this campaign by London's Metropolitan Police was an early April fool's joke. Sadly not.
This may be in London but my experiences with the police here in the U.S. tell me the sentiment is the same. So I encourage you all to go out there and take photographs. While the law allows still allows it.