It's actually a pretty big problem–there's pretty clear evidence that policemen serving in majority-black neighbourhoods tend to drift towards prejudice from workday to workday, only to somewhat recover on off-days. That's obviously a problem for the communities, but it's actually noticeable to the affected person as well, which really isn't enjoyable either.

What helps is to have people work as police officers, and paramedic or firefighter in turn. That way, you get to experience the good side of humanity and don't lose faith. But of course that makes training somewhat more complicated.