"If it is necessary for the police to go round in pairs, then just how safe is it on the street for everyone else?"
When I was young if an area was thought of as being rather dangerous for people to walk about in then it was said that the policemen always walked about in pairs.
Personally I doubt if the street are in fact as dangerous as are made out to be. Obviously there have always some parts of any large city in any country where you would be very foolish to venture alone in to late at night. But common-sense would tell you to stay out of such places at night. The danger in our streets is much more a case of how dangerous people perceive it to be rather than how dangerous they actually are.